2. NDT20 Course Notes

Radiographic Testing (RT) - Welds NDT20

Training and Examination Services Granta Park, Great Abington Cambridge CB21 6AL United Kingdom Copyright © TWI Ltd

Radiographic Testing (RT) - Welds Contents Section

Subject

Preliminary pages Contents Standards and Associated Reading COSHH, H&S, Cautions and Warnings Introduction to NDT Methods NDT Certification Schemes

1

Properties of Penetrating Radiation

2

The Electromagnetic Spectrum

3

Simple Atomic Theory

4

Ionising Radiation

5

X-rays or Bremsstrahlung

6

Gamma Rays

7

Methods of Producing a Radiographic Image

8

Production of a Radiograph (Film Radiography)

9

Sensitivity

5.1 6.1 6.2 6.3 6.4 6.5 6.6 7.1 7.2 8.1 8.2 8.3 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

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X-ray equipment

Alpha and beta emission Sealed sources Penetrating power of gamma radiation Quantity of gamma radiation Radioactive isotope containers for industrial radiography Comparison of X- and gamma rays Radiographic film Advanced imaging techniques

Radiographic quality Radiation scattering and scatter control Determining the correct exposure: Exposure charts Radiographic sensitivity Controlling radiographic quality EN ISO 19232-1 wire type IQIs Other wire type IQIs EN ISO 19232-2 Step-Hole type IQIs ASTM E 1025 plaque type penetrameters IQI sensitivity Duplex wire IQI

Copyright © TWI Ltd

10

10.1 10.2 10.3 10.4 10.5 10.6 10.7

11

11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8

Radiographic Techniques (for Welds in Plate and Pipe)

IQI type and placement Location markers Identification of radiographs Radiation energy Source to film distance SWSI techniques Double wall single image

Interpretation of Radiographs

Introduction Viewing conditions Reporting Film quality Interpretation of radiographic images Artefacts Interpretation of weld radiographs Interpretation of casting radiographs

12

Localisation

12.1 12.2 12.3

90ﾟmethod Tube (source) shift method Tube (source) shift method with lead markers

13

13.1 13.2 13.3 13.4 13.5 13.6 13.7

14

14.1 14.2

15

Units Used in Radiography

Ionisation (exposure) Absorbed dose Man mammal equivalent or radiobiological equivalent Dose rate Source strength or activity Specific activity Output

Radiation Monitoring Devices

Survey meters Personal monitors

Radiation Safety

15.1 15.2 15.3 15.4

Precautions Exposure limits for radiation workers Permitted levels Safe working distances

16

Glossary Appendix Product Technology Notes

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Preface These notes are provided as training reference material and to meet the study requirements for examination on the NDT course to which they relate. They do not form an authoritative document, nor should they be used as a reference for NDT inspection or used as the basis for decision making on NDT matters. The standards listed are correct at time of printing and should be consulted for technical matters. NOTE: These training notes are not subject to amendment after issue.

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Standards and Associated Reading EN ISO 1330-1

Non Destructive Testing – Terminology – Part 1: List of general terms

EN ISO 1330-2

Non Destructive Testing – Terminology – Part 2: Terms common to NDT methods

EN ISO 1330-3

Non Destructive Testing – Terminology – Part 3: Terms used in industrial radiographic testing

EN ISO 5579

Non-destructive testing – General principles for radiographic examination of metallic materials by X- and gamma-rays

EN ISO 19232-1

Non-destructive testing – Image quality of radiographs – Part 1: Image quality indicators (wire type) – Determination of image quality value

EN ISO 19232-2

Non-destructive testing – Image quality of radiographs – Part 2: Image quality indicators (step/hole type) – Determination of image quality value

EN ISO 19232-3

Non-destructive testing – Image quality of radiographs – Part 3. Image quality classes for ferrous metals

EN ISO 19232-4

Non-destructive testing – Image quality of radiographs – Part 4: Experimental evaluation of image quality values and image quality tables

EN ISO 19232-5

Non-destructive testing – Image quality of radiographs – Part 5. Image quality indicators (duplex wire type), determination of image unsharpness value

EN ISO 11699-1

Non-destructive testing – Industrial radiographic film – Part 1: Classification of film systems for industrial radiography

EN ISO 11699-2

Non-destructive testing – Industrial radiographic film – Part 2. Control of film processing by means of reference values

EN ISO 17636-1

Non-destructive testing of welds – Radiographic testing. Part 1: Xand gamma-ray techniques with film.

EN ISO 17636-2

Non-destructive testing of welds – Radiographic testing Part 2: Xand gamma ray techniques with digital detectors.

BS 4094-1

Recommendation for Data on shielding from ionizing radiation – Part 1: Shielding from gamma radiation

BS 4094-2

Recommendation for Data on shielding from ionizing radiation – Part 2: Shielding from X radiation

EN ISO 10675-1

Non-destructive testing of welds – Part 1: Evaluation of welded joints in steel, nickel, titanium and their alloys by radiography – Acceptance levels

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EN ISO 10675-2

Non-destructive testing of welds – Part 2: Evaluation of welded joints in aluminium and its alloys by radiography – Acceptance levels

EN 12681

Founding – Radiographic examination

EN 25580

Minimum requirements for industrial radiographic illuminators for non-destructive testing

BS M 34

Method of preparation and use of radiographic techniques

BS M 38

Guide to compilation of instructions and reports for the inservice non-destructive testing of aerospace products.

EN 4179

Aerospace series. Qualification and approval of personnel for non-destructive testing

EN ISO 9712

Non-destructive testing. Qualification and certification of personnel

Associated Reading NDT Ed.org – Introduction to radiographic testing http://www.ndted.org/EducationResources/CommunityCollege/Radiography/cc_rad_index.htm Mathematics and Formulae in NDT. Edited by Dr. R Halmshaw. Obtainable from the British Institute of Non-Destructive Testing

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COSHH, H&S, Caution and Warnings Relevant to TWI Training & Examination Services Introduction The use of chemicals in NDT is regulated by law under the Control of Substances Hazardous to Health (COSHH) Regulations 2005. These regulations require the School to assess and control the risk of health damage from every kind of substance used in training. Students are also required by the law to co-operate with the School’s risk management efforts and to comply with the Control Measures adopted. Hazard Data Sheets The School holds Manufacturers Safety Data Sheets for every substance in use. Copies are readily available for students to read before using any product. The Data Sheets contain information on:       

Trade name of the product; eg Magnaglo, Ardrox, etc. Hazardous ingredients of the products. Effect of those ingredients on people’s health. Hazard category of the substance; eg irritant, harmful, corrosive or toxic, etc. Special precautions for use; eg the correct personal protective equipment (PPE) to wear. Instructions for First Aid. Advice on disposal.

EH40 – Occupational Exposure Conditions 



Electrical Hazards include the following - Electrical shock and burns from contact with live parts - Injury from exposure to arcing or fire from faulty equipment - Explosion caused by electrical apparatus (or static electricity) - Electric shocks can lead to other types of injury such as falling from ladders or scaffolds. It is therefore important that workers know how to use electrical equipment and that it should be properly maintained and switched off when cleaning, adjusting or moving/transporting. As is the case with all items of test equipment and safety equipment, national regulations in the country of operation must be adhered to.

What is Exposure? Exposure to a substance is uptake into the body. The exposure routes are:    

Breathing fume, dust, gas or mist. Skin contact. Injection into the skin. Swallowing.

Many thousands of substances are used at work but only about 500 substances have workplace exposure limits (WELs). Until 2005 it had been normal for HSE to publish a new edition of EH40, or at least an amendment, each year. However, with increasing use of the website facilities, the HSE no longer always publishes a revised hardcopy edition or amendment. The web-based list applicable from 1 October http://www.hse.gov.uk/coshh/table1.pdf

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2007

can

now

be

found

at


Safety and Environmental Requirements Ultrasonic testing requires the use of couplant and cleaning fluids, some of which may be hazardous to health. Extended or repeated contact of such materials with the skin or mucous membranes shall be avoided. Testing materials shall be used in accordance with manufacturer’s instructions. National accident prevention, electrical safety handling of dangerous substances and personal and environmental protection regulations shall be observed at all times. Cautions and Warnings Some of the test samples used on the ultrasonic courses are heavy and become slippery when covered in couplant. Care should be taken when moving the samples and suitable PPE, particularly safety boots and barrier cream, should be used.

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Introduction to Non-Destructive Testing Non-destructive testing (NDT) is the ability to examine a material (usually for discontinuities) without degrading it or permanently altering the article being tested, as opposed to destructive testing which renders the product virtually useless after testing. Other advantages of NDT over destructive testing are that every item can be examined with no adverse consequences, materials can be examined for conditions internally and at the surface and, most importantly, parts can be examined whilst in service, giving a good balance between cost effectiveness and quality control. NDT is used in almost every industry with the majority of applications coming from the aerospace, power generation, automotive, rail, oil & gas, petrochemical and pipeline markets, safety being the main priority of these industries. When properly applied, NDT saves money, time, materials and lives. NDT as it is known today has been developing since around the 1920s, with the methods used today taking shape later and vast technological advancements being made during the Second World War. The basic principal methods are:      

Visual testing (VT). Penetrant testing (PT). Magnetic particle testing (MT). Eddy current testing (ET). Ultrasonic testing (UT). Radiographic testing (RT).

In all NDT methods, the interpretation of results is critical. Much depends on the skill and experience of the technician, although properly formulated test techniques and procedures will improve accuracy and consistency. Visual testing (VT) With sufficient lighting and access, visual techniques provide simple, rapid methods of testing whilst also being the least expensive. Close visual testing (CVT) refers to viewing directly with the eye (with or without magnification) whereas remote visual inspection (RVI) refers to the use of optical devices such as the boroscope and the fibrescope. Visual testing begins with the eye; however, the first boroscopes used a hollow tube and a mirror with a small lamp at the end to investigate the bores of rifles and cannons for problems and discontinuities. In the 1950s, the lamps were replaced by glass fibre bundles which were used to transmit the light. These became known as fibrescopes which were also less rigid, increasing the capabilities of testing. With usage expanding, many users began to suffer from eye fatigue which led to the development of video technology. This was first used in the 1970s and relies on electronics to transmit the images rather than fibreoptics. Further enhancements to video technology include pan, tilt and zoom lenses, and mounting cameras to platforms and wheels, all allowing more parts to be tested and better images for improved inspection. Video devices also allow recordings of inspections to be taken, meaning permanent records can be kept. This has a number of advantages such as enabling other inspectors to observe the test as it was performed and allowing further review and evaluation. Penetrant testing (PT) Penetrant testing locates surface-breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of excess penetrant, the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used.

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Advantages

Disadvantages

Applicable to non-ferromagnetics

Only detects defects open to the surface

Able to test large parts with a portable kit

Careful surface preparation required

Batch testing

Not applicable to porous materials

Applicable to small parts with complex geometry

Temperature dependent

Simple, cheap, easy to interpret

Cannot retest indefinitely

Sensitivity

Compatibility of chemicals

History of penetrant testing A very early surface inspection technique involved the rubbing of carbon black on glazed pottery. The carbon black would settle in surface cracks, rendering them visible. Later, it became the practice in railway workshops to examine iron and steel components by the oil and whiting method. In this method, heavy oil, commonly available in railway workshops, was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for ferromagnetic iron and steels. A different (though related) method was introduced in the 1940s. The surface under examination was coated with a lacquer, and after drying, the sample was caused to vibrate by the tap of a hammer. The vibration causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of stresses in a part and not for finding defects. Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in association with Switzer Bros, Cleveland, OH, USA. More effective penetrating oils containing highly visible (usually red) dyes were developed by Magnaflux to enhance flaw detection capability. This method, known as the visible or colour contrast dye penetrant method, is still used quite extensively today. In the 1940s, Magnaflux introduced the Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes referred to as black light), rendering indications from cracks and other surface flaws more readily visible to inspectors. UV lights have become increasingly portable with hand held UV torches now readily available.

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Magnetic particle testing (MT) Magnetic particle testing is used to locate surface and slightly sub-surface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material. Advantages

Disadvantages

Will detect some sub-surface defects

Ferromagnetic materials only

Rapid and simple to understand

Requirement to test in two directions

Pre-cleaning not as critical as with dye penetrant testing (PT)

Demagnetisation may be required

Will work through thin coatings

Oddly-shaped parts difficult to test

Cheap equipment

Not suited to batch testing

Direct test method

Can damage the component under test

History of magnetic particle testing The origins of MT can be traced to the 1860s when cannon barrels were tested for defects by first magnetising the barrel and then running a compass down the length of the barrel. By monitoring the needle of the compass, defects within the barrel could be detected. This form of NDT became much more common after the First World War, in the 1920s, when William Hoke discovered that flaws in magnetised materials created distortions in the magnetic field. When a fine ferromagnetic powder was applied to the parts, it was observed that they built up around the defects, providing a visible indication of their location. Magnetic particle testing superseded the oil and chalk method in the 1930s as it proved far more sensitive to surface breaking flaws. Today it is still preferred to the penetrant method on ferromagnetic material and much of the equipment being used then is very similar to that of today, with the only advances coming in the form of fluorescent coating to increase the visibility of indications and more portable devices being used. In the early days, battery packs and direct current were the norm and it was some years before alternating current proved acceptable. Magnetism The phenomenon called magnetism is said to have been discovered in the ancient Greek city of Magnesia, where naturally occurring magnets were found to attract iron. The use of magnets in navigation goes back to Viking times or maybe earlier, where it was found that rods of magnetised material, when freely suspended, would always point in a north-south direction. The end of the rod which pointed towards the North Pole star became known as the North Pole and consequently the other end became the South Pole. Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism, followed by Michael Faraday (1791-1867), whose experiments revealed that magnetic and electrical energy could be interchanged.

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Historical perspective Electromagnetic testing – the interaction of magnetic fields with circulating electrical currents - had its origin in 1831 when Michael Faraday discovered electromagnetic induction. He induced current flow in a secondary coil by switching a battery on and off. D E Hughes performed the first recorded eddy current test in 1879. He was able to distinguish between different metals by noting a change in excitation frequency resulting from effects of test material resistivity and magnetic permeability. Introduction to electromagnetic testing Many electromagnetic induction or eddy current comparators were patented in the period from 1952. Innumerable examples of comparator tests were reported in the literature and in patents. Many involved simple comparator coils into which round bars or other test objects were placed, producing simple changes in the amplitudes of test signals, or unbalancing simple bridge circuits. In nearly all cases, particularly where ferromagnetic test materials were involved, no quantitative analyses of test objects dimensions, properties, or discontinuities were possible with such instruments. Often, difficulties were encountered in reproducing test results. Some test circuits were adjusted or balanced to optimise signal differences between a known good test object and a known defective test object for each group of objects to be tested. Little or no correlation could then be obtained between various types of specimens, each type having been compared to an arbitrarily selected specimen of the same specific type. Developments in electromagnetic induction tests Rapid technological developments in many fields before and during the Second World War (1939-45) contributed both to the demand for NDT and to the development of advanced test methods. Radar and sonar systems allowed the viewing of test data on the screens of cathode-ray tubes or oscilloscopes. Developments in electronic instrumentation and magnetic sensors used both for degaussing ships and for actuating magnetic mines brought a resurgence of activity. Eddy current testing (ET) Eddy current testing is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor. Advantages

Disadvantages

Sensitive to surface defects

Very susceptible to permeability changes

Can detect through several layers

Only on conductive materials

Can detect through surface coatings

Will not detect defects parallel to surface

Accurate conductivity measurements

Not suitable for large areas and/or complex geometries

Can be automated

Signal interpretation required

Little pre-cleaning required

No permanent record (unless automated)

Portability

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History of eddy current testing The principles of eddy currents arose in 1831 with Faraday’s discovery of electromagnetic induction; eddy current testing methods have their origins in a period just after the First World War, when materials with a high magnetic permeability were being developed for electrical power transformer cores and motor armatures. Eddy currents are a considerable nuisance in electrical engineering – they dissipate heat and efforts to reduce their effect led to a discovery that they could be used to detect material changes and cracks in magnetic materials. The first eddy current testing devices for NDT were in 1879 by Hughes, who used the principles of eddy currents to conduct metallurgical sorting tests and the stray flux tube and bar tests. It was left to Dr Friedrich Förster in the late 1940s to develop the modern day eddy current testing equipment and formulate the theories which govern their use. The introduction by Förster of sophisticated, stable, quantitative test equipment and of practical methods for analysis of quantitative test signals on the complex plane was by far the most important factor contributing to the rapid development and acceptance of electromagnetic induction and eddy current testing. Förster is rightly identified as the father of modern eddy current testing. By 1950, he had developed a precise theory for many basic types of eddy current tests, including both absolute and differential or comparator test systems and probe or fork coil systems used with thin sheets and extended surfaces. Continued advances in research and development, advanced electronics and digital equipment have led to eddy currents becoming one of the most versatile of the surface methods of inspection. Eddy current methods have developed into a wide range of uses and are recognised as being the forerunner of NDT techniques today. From the mid1980s, microprocessor-based eddy current testing instruments were developed which had many advantages for inspectors. Modern electronics have made instruments more user friendly, providing reduced noise levels which made certain test applications very difficult, but also improving methods of signal presentation and recording capabilities. Applications for microcomputer chips abound, from giving lift-off suppression in simple crack detection to providing signal processing for immediate analysis of condenser tube inspection. As with other testing methods, improvements to the equipment have been made to increase its portability and computer-based systems now allow easy data manipulation and signal processing. Eddy current testing is now a widely used and understood inspection method for flaw detection as well as for thickness and conductivity measurements. Ultrasonic testing (UT) Ultrasonic testing measures the time for high frequency (0.5-50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound will return to the probe in a time period other than that expected of a faultfree specimen.

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Advantages

Disadvantages

Sensitive to cracks at various orientations

No permanent record (unless automated)

Portability

Not easily applied to complex geometries and rough surfaces

Safety

Unsuited to coarse grained materials

Able to penetrate thick sections

Reliant upon defect orientation

Measures depth and through-wall extent

History of ultrasonic testing In Medieval times craftsmen casting bells for churches were aware that a properly cast bell rang true when struck and that a bell with flaws would give out a false note. This principle was used by wheel-tappers inspecting rolling stock on the railways; they struck wheels with a hammer and listened to the note given out. A loose tyre sounded wrong. The origin of modern ultrasonic testing (UT) is the discovery by the Curie brothers in 1880 that quartz crystals cut in a certain way produce an electric potential when subjected to pressure - the piezo-electric effect, from the Greek piedzein (to press or strike). In 1881 Lippman theorised that the effect might work in reverse, and that quartz crystals might change shape if an electric current was applied to them. He found that this was so and experimented further. Crystals of quartz vibrate when alternating currents are applied to them. Crystal microphones in a modern stereo rely on this principle. When the Titanic sank in 1912, the Admiralty tried to find a way of locating icebergs by sending out sound waves and listening for an echo. They experimented further with sound to detect submarines during the First World War. Between the wars, marine echo sounding was developed and in the Second World War ASDIC (Anti-Submarine Detection Investigation Committee) was extensively used in the Battle of the Atlantic against the U-boats. In 1929, the Russian physicist Sokolov experimented with through-transmission techniques, passing vibrations through metals to find flaws; this work was taken up by the Germans. In the 1930s the cathode ray tube was developed and miniaturised in the Second World War to fit small airborne radar sets into aircraft. It made the UT set as we know it possible. Around 1931 Mulhauser obtained a patent for a system using two probes to detect flaws in solids and following this Firestone (1940) and Simons (1945) developed pulsed UT using a pulse-echo technique. In the years after the Second World War, researchers in Japan began to experiment on the use of ultrasound for medical diagnostic purposes. Working largely in isolation until the 1950s, the Japanese developed techniques for the detection of gallstones, breast masses, and tumours. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation. The first flaw detector was made by Sproule in 1942 while he was working for the Scottish firm Kelvin & Hughes. Similar work was carried out by Firestone in the USA and by German physicists. Sproule went on to develop the shear-wave probe.

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Initially UT was limited to testing aircraft, but in the 1950s it was extensively used in the building of power stations in Britain for examining thick steel components safely and cheaply. UT was found to have several advantages over radiography in heavy industrial applications:    

No health hazards were associated with radiography, and a UT technician could work next to welders and other employees without endangering them of holding up work. It was efficient in detecting toe cracks in boilers – a major cause of explosions and lack of fusion in boiler tubes. It could find planar defects, like laminations, which were sometimes missed by radiography. A UT check on a thick component took no more time than a similar check on a thin component as opposed to long exposure times in radiography.

Over the next twenty years, improvements focused on accurate detection and sizing of the flaws with limited success, until 1977 when Silk first discovered an accurate measurement and display of the top and bottom edges of a discontinuity with the timeof-flight diffraction (TOFD) technique. Advances in computing technology have now expanded the use of TOFD as real time analyses of results are now available. It was also during the 1970s that industries focused on reducing the size and weight of ultrasonic flaw detectors and making them more portable. This was achieved by using semiconductor technology and during the 1990s microchips were introduced into the devices to allow calibration parameters and signal traces to be stored. LCD display panels and digital technology have also contributed to reducing the size and weight of ultrasonic flaw detectors. With the development of ultrasonic phased array and increased computing power, the future for ultrasonic inspection is very exciting. Ultrasound used for testing The main use of ultrasonic inspection in the human and the animal world is for detecting objects and measuring distance. A pulse of ultrasound (a squeak from a bat or a pulse from an ultrasonic source) hits an object and is reflected back to its source like an echo. From the time it takes to travel to the object and back, the distance of the object from the sound source can be calculated. That is how bats fly in the dark and how dolphins navigate through water. It is also how warships detected and attacked submarines in the Second World War. Wearing a blindfold, you can determine if you are in a very large hall or an ordinary room by clapping your hands sharply; a large hall will give back a distinct echo, but an ordinary room will not. A bat’s echo location is more precise: the bat gives out and can sense short wavelengths of ultrasound and these give a sharper echo than we can detect. In UT a sound pulse is sent into a solid object and an echo returns from any flaws in that object or from the other side of the object. An echo is returned from a solid-air interface or any solid-non-solid interface in the object being examined. We can send ultrasonic pulses into material by making a piezo-electric crystal vibrate in a probe. The pulses can travel in a compression, shear or transverse mode. This is the basis of ultrasonic testing. However, the information from the returning echoes must be presented for interpretation. It is for this purpose that the UT set, or flaw detector as it is frequently called, contains a cathode ray tube. In the majority of UT sets, the information is presented on the screen in a display called the A Scan. The bottom of the CRT screen is a time base made to represent a distance say 100mm. An echo from the backwall comes up on the screen as a signal, the amplitude of which represents the amount of sound returning to the probe. By seeing how far the signal comes along the screen we can measure the thickness of the material we are examining.

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If that material contains a flaw, sound is reflected back from the flaw and appears on the screen as a signal in front of the backwall echo (BWE) as the sound reflected from the flaw has not had so far to travel as that from the backwall. BWE BWE

BWE BWE

Defect Defect

Ultrasonic signals Anything that sends back sound energy to a probe to cause a signal on the screen is called a reflector. By measuring the distance from the edge of the CRT screen to the signal, we can calculate how far down in the material the reflector lies. Radiographic testing (RT) Radiography monitors the varying transmission of ionising radiation through a material with the aid of photographic film or fluorescent screens to detect changes in density and thickness. It will locate internal and surface-breaking defects. Advantages

Disadvantages

Gives a permanent record, the radiograph

Radiation health hazard

Detects internal flaws

Can be sensitive to defect orientation and so can miss planar flaws

Detects volumetric flaws readily

Limited ability to detect fine cracks

Can be used on most materials

Access is required to both sides of the object

Can check for correct assembly

Skilled radiographic interpretation is required

Gives a direct image of flaws

Relatively slow method of inspection

Fluoroscopy can give real time imaging

High capital cost High running cost

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History of radiographic testing X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wϋrzburg University in Germany. Whilst performing experiments in which he passed an electric current through a Crookes tube (an evacuated glass tube with an anode and a cathode), he found that when a high voltage was applied, the tube produced a fluorescent glow. Roentgen noticed that some nearby photographic plates became fogged. This caused Roentgen to conclude that a new type of ray was being emitted from the tube. He believed that unknown rays were passing from the tube and through the plates. He found that the new ray could pass through most substances. Roentgen also discovered that the ray could pass through the tissue of humans, but not bones and metal objects. One of Roentgen's first experiments late in 1895 was a film of the hand of his wife. Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896 French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays, and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. It was Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with photographic plates. Later when he developed these plates, he discovered that they were fogged (indicating exposure to light). Becquerel wondered what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light; in addition, he noticed that only the plates that were in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. Becquerel did not pursue his discovery of radioactivity, but others did. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitch-blende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements.

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In 1898, the Curies discovered another radio-active element in pitchblende, and named it polonium in honour of Marie’s native homeland. Later that year, the Curies discovered another radioactive element which they named ‘radium’, or shining element. Both polonium and radium were more radioactive than uranium. Due to her lifelong research in this field, Marie Curie is widely credited with the discovery of gamma radiation and the introduction of the new term: radio-active. Since these discoveries, many other radioactive elements have been discovered or produced. Radiography in the form of NDT took shape in the early 1920s when H H Lester began testing on different materials. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. During the Second World War, industrial radiography grew tremendously as part of the Navy's shipbuilding programme. In 1946, man-made gamma ray sources from elements such as cobalt and iridium became available. These new sources were far stronger than radium and much less expensive. The man-made sources rapidly replaced radium, and the use of gamma rays increased quickly in industrial radiography. William D Coolidge's name is inseparably linked with the X-ray tube popularly called the Coolidge tube. This invention completely revolutionised the generation of Xrays and remains the model upon which all X-ray tubes for medical applications are patterned. He invented ductile tungsten, the filament material still used in such lamps. He was awarded 83 patents. Although the theories and practices have changed very little, radiographic equipment has developed. These developments include better images through higher quality films and also lighter, more portable equipment. In addition to conventional film radiography, digital radiographic systems are now widespread within the NDT industry. The use of photostimulable phosphor (PSP) bearing imaging plates with photomultipliers to capture image signals and analogue-to-digital converters (ADC) are used extensively in computed radiography (CR). Direct radiography (DR) systems are also used based upon complementary metal oxide sensor (CMOS) technology and TFT (thin film transistors). These systems have the ability to directly convert light into digital format; additionally, they may be coupled with a scintillator which coats CMOS and charged couple device (CCD) sensors. The scintillator converts photon energy to light before the sensor and ADC converts to digital format. Systems which use scintillators in this way are often referred to as indirect systems. Quality issues of any digital system are based upon the effective pixel size and the signal-to-noise ratio (SNR). The benefits of using digital systems are the speed of inspection and the absence of chemical processing requirements and wet film; however, the initial equipment costs will be high.

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NDT Certification Schemes CSWIP – Certification Scheme for Personnel Managed by TWI Certification Ltd (TWICL), a TWI Group company formed in 1993 to separate TWI’s activities in the field of personnel and company certification thus ensuring continued compliance with international standards for certification bodies and is accredited by UKAS to BS EN ISO 17024. TWICL establishes and implements certification schemes, approves training courses, and authorises examination bodies and assessors in a large variety of inspection fields, including; non-destructive testing (NDT), welding and plant inspectors, welding supervisors, welding coordination, plastic welders, underwater inspectors, integrity management, general inspection of offshore facilities, cathodic protection, heat treatment. TWI Certification Ltd Granta Park, Great Abington, Cambridge CB21 6AL, United Kingdom Tel: +44 (0) 1223 899000 Fax: +44 (0) 1223 894219 Email: [email protected] Website: www.cswip.com

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PCN – Personal Certification in Non-destructive testing Managed and marketed by the British Institute of Non-Destructive Testing (BINDT) which owns and operates the PCN Certification Scheme, it offeres a UKAS accreditied certification of competence for NDT and condition monitoring in a variety of product sectors. The British Institute of Non-Destructive Testing Certification Services Division, Newton Building, St. Georges Avenue, Northampton, NN2 6JB, United Kingdom Tel: +44 (0)1604 893811 Fax: +44 (0)1604 892861 Email: [email protected] Website: http://www.bindt.org/Certification/General_Information Both schemes offer NDT certification conforming to BS EN ISO 9712; Qualification and Certification of NDT personnel, this superseding EN473. The PCN Scheme What follows is a summary of the general requirements for qualification and PCN certification of NDT personnel as described in PCN/GEN Issue 5 Revision R. PCN Certification is a scheme which covers the qualification of NDT inspection staff to meet the requirements of European and International Standards. Typically a standard or procedure will call for the Inspector to be certified in accordance with BS EN ISO 9712 and/or PCN requirements. The PCN Gen Document describes how the PCN system works. The points below cover extracts from this document which are major items, the full document can be viewed on the BINDT website – www.bindt.org/certification/PCN.

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References PCN documents PSL/4 PSL/8A PSL/30 PSL/31 PSL/42 PSL/44 PSL/49 PSL/51 PSL/57C PSL/67 PSL/70 CP9 CP16 CP17 CP19 CP22 CP25 CP27

Examination availability PCN documents – issue status Log of pre-certification experience Use of PCN & UKAS logo Log of pre-certification on-the-job training Vision requirements Examination exemptions for holders of certification other than PCN Acceptable certification for persons supervising PCN candidates gaining experience prior to certification Application for certification, experience gained post examination Supplementary 56 day waiver Request for L2 certificate issue to a L3 holder Requirements for BINDT authorised qualifying bodies Renewal and recertification of PCN Levels 1 & 2 certificates Renewal and recertification of PCN Level 3 certificates Informal access to authorised qualifying bodies by third parties Marking and grading PCN examinations Guidelines for the preparation of NDT procedures and instructions in PCN examinations Code of ethics for PCN certificate holders

PCN/GEN Appendix Z1 – NDT Training Syllabi Levels of PCN certification Level 1 personnel are qualified to carry out NDT operations according to written instructions under the supervision of appropriately qualified Level 2 or 3 personnel. Within the scope of the competence defined on the certificate, Level 1 personnel may be authorised by the employer to perform the following in accordance with NDT instructions:    

Set up equipment. Carry out the test. Record and classify the results in terms of written criteria. Report the results.

Level 1 personnel have not demonstrated competence in the choice of test method or technique to be used, nor for the assessment, characterisation or interpretation of test results.

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Level 2 personnel have demonstrated competence to perform and supervise nondestructive testing according to established or recognised procedures. Within the scope of the competence defined on the certificate, Level 2 personnel may be authorised by the employer to:          

Select the NDT technique for the test method to be used. Define the limitations of application of the testing method. Translate NDT standards and specifications into NDT instructions. Set up and verify equipment settings. Perform and supervise tests. Interpret and evaluate results according to applicable standards, specifications. Prepare written NDT instructions. Carry out and supervise all Level 1 duties. Provide guidance for personnel at or below Level 2. Organise and report the results of non-destructive tests.

codes

or

Level 3 personnel are qualified to direct any NDT operation for which they are certificated and may be authorised by the employer to:      

Assume full responsibility for a test facility or examination centre and staff. Establish, review for editorial and technical correctness and validate NDT instructions and procedures. Interpret codes, standards, specifications and procedures. Designate the particular test methods, techniques and procedures to be used. Within the scope and limitations of any certification held carry out all Level 1 and 2 duties and; Provide guidance and supervision at all levels.

Level 3 personnel have demonstrated:  



Competence to interpret and evaluate test results in terms of existing codes, standards and specifications. Possession of the required level of knowledge in applicable materials, fabrication and product technology sufficient to enable the selection of NDT methods and techniques and to assist in the establishment of test criteria where none are otherwise available. General familiarity with other NDT methods.

Level 3 certificated personnel may be authorised to carry out, manage and supervise PCN qualification examinations on behalf of the British Institute of NDT. Where Level 3 duties require the individual to apply routine NDT by a method(s) within a particular product or industry sector, the British Institute of NDT strongly recommends that industry demand that this person should hold and maintain Level 2 certification in the applicable methods and sectors.

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Training Table 1 Minimum required duration of training. NDT method

Level 1 hours

Level 2 hours1

Level 3 hours

ET

40

40

40

PT

16

24

24

MT

16

24

32

RT

40

80

72

RI

N/A

56

N/A

UT

40

80

72

VT

16

24

24

BRS

16

N/A

N/A

RPS

N/A

24

N\A

Basic knowledge

(Direct access to Level 3 examination parts A- C)

80

Note 1. Direct access to Level 2 requires the total number of hours shown in Table 1 for Levels 1 and 2, and direct access to Level 3 requires the total number of hours shown in Table 1 for Levels 1-3. Up to one third of the total specified in this table may take the form of OTJ training documented using form PSL/42 provided it is verifiable and covered practical application of the syllabus detailed in CEN ISO/TR 25107:2006.

Industrial NDT experience   

Industrial NDT experience in the appropriate sector may be acquired prior to or following success in the qualification examination. In the event that the experience is sought following successful examination, the results of the examination shall remain valid for up to two years. Documentary evidence (in a form acceptable to the British Institute of NDT, i.e. on PCN form PSL/30) of experience satisfying the following requirements shall be confirmed by the employer and submitted to BINDT AQB prior to examination, or directly to BINDT prior to the award of PCN certification in the event that experience is gained after examination.

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Table 2 Minimum duration of experience for certification. Experience, months NDT method

Level 1

Level 2

Level 3

ET

3

9

18

MT

1

3

12

PT

1

3

12

RT

3

9

18

UT

3

9

18

RI

N/A

6

N/A

VT

1

3

12

Work experience in months is based on a nominal 40-hour week or the legal week of work. When an individual is working in excess of 40h/week, he may be credited with experience based on the total hours, but he shall be required to produce evidence of this experience. Direct access to Level 2 requires the total number of hours shown in Table 2 for Levels 1 and 2, and direct access to Level 3 requires the total number of hours shown in Table 2 for Levels 1-3

Qualification examination Table 3 Numbers of general questions. NDT method

Level 1

Level 2

ET

40

40

PT

30

40

MT

30

40

RT

40

40

RI

N/A

40

UT

40

40

VT

30

40

BRS

30

N/A

RPS

N/A

20 plus 4 narrative

Note:

All Level 1 specific theory papers have 30 questions. All Level 2 specific theory papers have 36 questions.

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Re-examination a

A candidate who fails to obtain the pass grade for any examination part (general, specific or practical) may be re-examined twice in the failed part(s), provided the reexamination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination.

b

A candidate who achieves a passing grade of 70% in each of the examination parts (general, specific or practical) but whose average score is less than the required 80% may be re-examined a maximum of two times in any or all of the examination parts in order to achieve an overall average score of 80%, provided the re-examination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination.

c

A candidate who fails all permitted re-examinations shall apply for and take the initial examination according to the procedure established for new candidates.

d

A candidate whose examination results have not been accepted for reason of fraud or unethical behaviour shall wait at least twelve months before re-applying for examination.

Summary The PCN scheme is managed and administered by the British Institute of NDT (BINDT) on behalf of its stakeholders. It meets or exceeds the criteria of BS EN ISO 9712. There are 6 appendices covering various industry and product sectors, 1 2 3 4 5 6

Aerospace. Castings. Welds. Wrought Products and Forgings. Pre and in-service inspection (multi sector). Railway.

There are many additional supporting documents varying from vision requirements PSL44 to renewal and recertification (Levels 1 and 2 – CP16; Level 3 – CP17) and so on. The document defines many terms used in certification of NDT personnel (PCN Gen Section 3) The certification body (BINDT) meets the requirements of BS EN ISO 17024 (PCN Gen section 5)

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BINDT approves authorised qualifying bodies (AQBs) to carry out the examinations (PCN Gen Section 5) a b c d e f g h i j k l

The document sets out the Levels of PCN certification and what each level of personnel is qualified to do (PCN Gen section 6). There are 3 Levels of PCN certification. Candidates for examination must have successfully completed a BINDT validated course of training at a BINDT authorised training organisation (PCN Gen Section 7). Table 1 shows the minimum required duration of training for all Levels and methods plus a section of notes. Table 2 gives the minimum duration of experience for each Level and method. A candidate is required to have a vision test of colour perception and a near vision test (Jaeger Number 1 or N4.5). PCN Gen Section a – the near vision test to be taken annually. Examination applications are made directly with the AQB. PCN Level 1s and 2 initial exams comprise general; specific and practical parts. Table 3 shows the number of general questions at Levels 1 and 2 examinations. There are 30 specific questions on the Level 1 papers. There are 36 questions on the Level 2 specific papers. A variety of practical samples are tested depending on the method and sector. A Level 3 examination comprises a basic and a method examination – however the basic examination needs to be passed only once. Table 4 shows the number of basic examination questions. Table 5 shows the number of Level 3 examination questions.

Table 4 Number of basic examination questions. Part

Examination

Number of questions

A

Materials technology and science, including typical defects in a wide range of products including castings welds and wrought products.

30

B

Qualification and certification procedure in accordance with this document

10

C

15 general questions at Level 2 standard for each of four NDT methods chosen by the candidate, including at least one volumetric NDT method (UT or RT).

60

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Table 5 Main method examination. Part

Subject

Number of questions

D

Level 3 knowledge relating to the test method applied

30

E

Application of the NDT method in the sector concerned, including the applicable codes, standards, and specifications. This may be an open book examination in relation to codes, standards, and specifications.

20

F

Drafting of one or more NDT procedures in the relevant sector. The applicable codes, standards, and specifications shall be available to the candidate.

m A pass is obtained where each part is 70% or over with an average grade of 80% or over. n A PCN certificate is valid for 5 years. o Renewal and recertification requirements are covered in CP16 for Level 1 and Level 2 and CP17 for Level 3.

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Section 1 Properties of Penetrating Radiation (X- and Gamma)

1

Properties of Penetrating Radiation (X- and Gamma) Radiation has six basic properties: 1

Can penetrate material (therefore penetrating radiation).

2

Can ionize matter (therefore ionizing radiation).

3

Propagates in a straight line (rectilinear propagation)

4

Can cause fluorescence of some materials.

5

Can cause chemical effects.

6

Has physiological effect.

Penetrating radiation can be used in non-destructive examination (NDE) because it travels in a straight line and may be absorbed as it passes through matter. The extent to which it is absorbed depends upon three factors: 1 2 3

Thickness of the absorber. Physical characteristics of the absorber (in particular its density and atomic number). Wavelength or photon energy of the radiation itself.

Penetrating radiation can be detected using a photographic emulsion or by other means. The system used to detect the radiation must be capable of differentiating between different intensities of radiation. Film

Object Source of radiation

Figure 1.1 Penetrating radiation passing through an object and the resulting radiograph.

Note: In film radiography thin sections appear darker while thicker sections appear lighter. The opposite is true if a fluorescent screen rather than a photographic film is used as a radiation detector.

NDT20-71015 Properties of Penetrating Radiation

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Object

Film

Source of radiation

Figure 1.2 Radiation passing through an object containing two voids at different depths and the resulting radiograph.

Two important points to keep in mind when viewing a radiographic image are: 1

A radiograph is a two-dimensional image of a three-dimensional object: The through thickness position and size of an object producing a radiographic image cannot be determined solely from the information given by a single radiograph (demonstrated in Figure 1.2).

2

A defect will only appear as an image in a radiograph if the:  

Defect causes a local difference in radiation absorption. Method used for detecting the radiation is capable of detecting the difference in radiation intensity so caused.

For example, suppose that a chosen radiographic technique is capable of detecting a thickness difference of say 0.5mm in 50mm of steel. If we use this technique to radiograph the weld shown in Figure 1.3 then: 1

The gas pore will readily be detected because

2

The lack of side fusion will not appear as an image on the radiograph because 0.01 which is much too small to be detected by the technique used.


1-2

3

.


Set-up

Resultant radiograph

Figure 1.3 Radiography of a weld.


1-3


Section 2 The Electromagnetic Spectrum

2

The Electromagnetic Spectrum From the early part of the nineteenth centrury it was understood that light was a waveform. Light, however, was well known to be capable of passing through a vacuum. Scientists of the day puzzled long and hard as to how it could be possible that a waveform could travel without some form of matter to support it, therefore the concept of a substance called ether, which filled otherwise apparently empty space, was postulated and a lot of research time was expended in trying to isolate this mysterious substance. The search continued until around 1865 when a scientist called James Maxwell predicted the existence of electromagnetic waves. Such waves, he said, would be capable of passing through a vacuum, since they were supported by oscillating magnetic and electrical fields mutually at right angles to each other and to the direction of propagation. Moreover, using mathematics, Maxwell predicted a speed of travel for such waves that was equal to the then known speed of light. It soon became clear that light was in itself a form of electromagnetic radiation. All types of electromagnetic radiation travel at the same velocity ( ), the velocity of light, which is about 2.998 x 108ms-1 (ca. 300,000 km/s or 186,000 miles per second), but differ in terms of their wavelength ( ) and frequency ( ). Wavelength can be defined as the distance travelled during one complete field oscillation while frequency is the total number of oscillations occurring in one second.

∙

As scientific knowledge advanced it became clear that in some circumstances light behaved not so much like a waveform, but more like a particle. Considering such behaviour in 1900 a scientist, Max Planck, first put forward the theory that light had, what he called, a quantum nature. Planck postulated that electromagnetic energy could not exist in amounts (quantum being Latin for amount) smaller than a given very small amount of energy and that all larger amounts of electromagnetic energy were exact multiples of this amount to which he gave the name photon. Planck believed that the photon energy of any form of electromagnetic radiation would be equal to a constant multiplied by its frequency. In later years Planck’s hypothesis was proved to be true and the constant in question became known as Planck’s constant, usually abbreviated as .

∙

Where h is Planck’s constant (= 6.63 x 10–23Js) and electromagnetic radiation of frequency .

is the photon energy of

The properties of electromagnetic radiation, especially in the way it interacts with matter are largely determined by its wavelength. Figure 2.1 shows the electromagnetic spectrum.

NDT20-71015 The Electromagnetic Spectrum

2-1


Wave length in nanometres

Photon energy in MeV Figure 2.1 Electromagnetic spectrum

The electron volt (eV) is a unit of energy equal to the kinetic energy that an electron obtains when it accelerates through an electric field of 1V. One electron volt is equal to 1.6 x 10-19 Joules. A Mega electron volt (MeV) is equal to the kinetic energy of an electron that has accelerated through an electric field of 1 million volts (see Figure 2.1). From Figure 2.1 also outcomes that as wavelength increase the energy decreases and vice versa. This relation reflects the formula ∙ . This fact is important when penetration ability of radiation is considered. Photon needs to have more energy to penetrate dense material (eg lead) in comparison to case when aluminium (light metal) is to be penetrated. Note: The relationship between wavelength and photon energy on which the diagram above has been based is approximate.

NDT20-71015 The Electromagnetic Spectrum

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Section 3 Simple Atomic Theory

3

Simple Atomic Theory To understand how X- and gamma rays are produced it is necessary to have a basic understanding of atomic theory. An atom is the smallest part of any chemical element. Atoms are known to consist of three basic types of particle, these being the positively charged proton, the neutron (which has no electrical charge) and the negatively charged electron. The electrical charge on the proton and electron are equal in magnitude but opposite in polarity. The atomic mass of a proton is, by definition, equal to 1 atomic mass unit (abbreviation: amu; 1 amu = 1.6725 x 10-27kg). The electron has a tiny mass, around 1/1836 that of a proton (0.000545 amu or about 9.11 x 10-31kg), while that of a neutron is very slightly greater than that of a proton at 1.0014 amu (or 1.6748 x 10-27kg). The atom is thought to consist of a positively charged nucleus (which consists of protons and neutrons) surrounded by a cloud of orbiting negatively charged electrons. Shell consists of electrons Nucleus consists of Protons + neutrons

Figure 3.1 Simple model of atomic structure.

In the equilibrium state the number of orbital electrons is equal to the number of protons and there is no net electrical charge. When there is inequality between the number of protons and electrons then there is a net electrical charge and the atom is said to be ionised. Ions may be negatively charged if the number of electrons exceeds the number of protons or positively charged if the converse is true. So called electropositive elements, a group which includes all metals, like to form positive ions while the electronegative elements such as oxygen, phosphorus, chlorine and sulphur like to form negative ions. The orbital electrons exist in fixed energy levels or shells. Each shell can contain a fixed maximum number of electrons. The shells are identified by letters – K, L, M, N and so on. The lowest energy level is represented by the K-shell; this is the innermost of the electron shells and it can contain a maximum of two electrons. The L-shell can contain up to eight electrons while the M-shell contains a maximum of 18 and the N-shell contains a maximum of 32. The maximum total number of electrons in each shell is equal to 2n2 where n is the shell number counting the K-shell as one, L-shell as two, etc. Within the M, N and other shells certain groupings of electrons produce greater stability, elements having an even number of electrons tend to less chemically reactive than those which have an odd number. A group of eight electrons in the M or N shells produces an element which is the most chemically inert of all elements – an inert gas. In electropositive elements the orbital electrons are relatively loosely bound and there is a tendency to form positive ions. In electronegative elements the orbital electrons are relatively tightly bound and there is a tendency to form negative ions. The inert gases such as neon, argon, xenon and krypton either have an outer shell that is completely full or one which contains a very stable grouping of electrons. Based on their chemical properties the elements can be organised into a periodic table as shown below. Elements falling in the same vertical column share very similar chemical properties.

NDT20-71015 Simple Atomic Theory

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1

2

H

He

1.008 3

Li

6.940 11

Na

4

5

Be

B

9.012 12

10.81 13

Mg

Al

22.99 19

24.30 20

21

22

23

24

25

26

27

28

29

30

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

39.10 37

Rb

85.47 55

Cs

132.9 87

Fr

(223)

40.08 38

Sr

87.62 56

Ba

137.3 88

Ra

226.0

44.96 39

Y

88.91 57

La

138.9 89

47.90 40

Zr

91.22 72

Hf

178.5

50.94 41

Nb

92.91 73

Ta

181.0

52.00 42

Mo

95.94 74

W

183.9

54.94 43

Tc

98.91 75

Re

186.2

55.85 44

Ru

101.1 76

Os

190.2

58.93 45

Rh

102.9 77

Ir

192.2

58.71 46

Pd

106.4 78

Pt

195.1

63.55 47

Ag

107.9 79

Au

197.0

65.38 48

Cd

112.4 80

Hg

200.6

26.98 31

Ga

69.74 49

In

114.8 81

Tl

204.4

6

C

12.01 14

Si

28.09 32

Ge

72.59 50

Sn

118.7 82

Pb

207.2

7

N

14.01 15

P

30.97 33

As

74.92 51

Sb

121.8 83

Bi

209.0

8

O

16.00 16

S

32.06 34

Se

78.96 52

Te

127.6 84

Po

(209)

9

F

19.00 17

Cl

35.45 35

Br

79.90 53

I

126.9 85

At

(210)

4.003 10

Ne

20.17 18

Ar

39.95 36

Kr

83.80 54

Xe

131.3 86

Rn

(222)

Ac

(227)

Lanthanide Series Actinide Series

58

Ce

140.1 90

Th

232.0

59

Pr

60

Nd

140.9

144.2

91

92

231.0

238.0

Pa

U

61

Pm (145) 93

Np

237.1

62

Sm

150.4 94

Pu

(244)

63

Eu

64

Gd

152.0

157.3

95

96

(243)

(249)

Am Cm

65

Tb

158.9 97

Bk

(247)

66

Dy

162.5 98

Cf

(251)

67

Ho

164.9 99

Es

(254)

68

Er

167.3 100

Fm

(257)

69

Tm

168.9 101

Md

(258)

70

Yb

173.0 102

No

(259)

71

Lu

175.0 103

Lw

(260)

Figure 3.2 Periodic table of elements.

The numbers above and below each chemical symbol are the atomic number and the atomic weight of each element. Note: That the atomic weight differs slightly from the atomic mass number. X-rays result from ionisation and de-ionisation events: when a positively charged ion captures a free electron the atom descends into a lower energy state and the left over energy may be released in the form of an X-ray photon. X-rays can also result as a negative ion loses a captured electron, because again there is a reduction in the energy stored in the atom as it returns to a state of zero electrical charge. Each element has its own characteristic number of protons in the nucleus. This number is the atomic number, usually abbreviated as Z. It is the atomic number that determines the chemical properties of a given substance. However, each element can exist as any one of a number of nuclides or isotopes. Each isotope of a given element has the same atomic number, number of protons and chemical properties, but each isotope has a different atomic mass number.


3-2


The difference in atomic mass number is due to a difference in the number of neutrons in the nucleus. The atomic mass number is equal to the total of protons + neutrons in the nucleus. Most elements can exist in nature as any one of several stable isotopes. Some isotopes, however, are not stable – these are the so called radioactive isotopes. The following notation is typically used, for example:

Where: 59 is the number of protons + neutrons (the atomic mass number). 27 is the number of protons (the atomic number). is the chemical symbol, in this case cobalt. The example shown, if in a non-ionised state, would have 27 protons, 27 electrons and 32 neutrons in each atom. Radioactive isotopes undergo fission as they decay towards a more stable atomic structure. Some isotopes achieve stability in a single fission event while others may undergo a series of fission events before the result is a stable isotope or combination of stable isotopes. Some radioactive isotopes can decay along any one of several decay paths. Each individual fission event is random; however, each unstable atom of a given isotope has the same probability of decay. One atom of an isotope may undergo fission with a life of a few microseconds while its neighbour may not decay until several centuries have passed, but both radioactive atoms still have the same probability of decay. Applied to a very large number of active atoms, and a typical iridium 192 gamma ray source [as used in industrial radiography], despite its small physical size, may contain around 1020 radioactive atoms (that’s one hundred million million million), the constant probability of decay gives rise to a constant halflife. With such a large number the random nature of decay just averages out. Gamma rays are an occasional by product of this process of nuclear fission. Fission means splitting. There are several routes by which nuclear fission can take place and two of these are of importance in the production of gamma rays. These will be discussed in greater detail in Section 7.


3-3


Section 4 Ionising Radiation

4

Ionising Radiation The two types of penetrating radiation most used in industrial radiography, Xand gamma rays, are often referred to as ionising radiation. This is because the nature of their interaction with matter is to cause ionisation. Ionisation is caused by loss of an orbiting electron which leaves the atom in an electrically positively charged state (+ ion). Alpha and beta particles, which are products of radioactive fission also cause ionisation and are therefore included within the term ‘ionising radiation’. Neutron radiation is a hazard in the nuclear power industry, it can [indirectly] cause ionisation, and it is therefore often included within this group of types of radiation referred to as ionising. Alpha and beta particle radiation are covered in greater detail in Sections 6 and 7.

NDT20-71015 Ionising Radiation

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Section 5 X-Rays or Bremsstrahlung

5

X-Rays or Bremsstrahlung The term X-ray is applied to ionising radiation produced when a beam of high velocity (ie high kinetic energy) electrons collides with the atoms of a target material. The photon energy of the X-radiation thereby produced depends on two factors: 1 2

Kinetic energy of the electron at the point of collision. Relative efficiency of the process of stopping the incident electron – does this occur in a single large event or in a series of events of varying magnitude? Kinetic energy of the electron partially absorbed, low energy X-ray photon emitted X-ray photon (low energy) Deflected electron Atom in target material

High velocity electrons

X-ray photon (high energy)

Captured electron Kinetic energy of the electron completely absorbed, high energy X-ray photon emitted Figure 5.1 X-ray production.

The maximum energy of the X-ray photons produced is determined by the maximum kinetic energy of the high velocity electrons impacting upon the target material. There is no minimum to the energy of the X-ray photons produced, because there is wide variation in the amount of energy which the electron loses on collision with an atom. Some of the electrons will score only a glancing hit on the atom, in so doing interacting with the loosely bound electrons in the outermost electron shells. This causes the impacting electron to be deflected and it loses part of its velocity. The reduced energy electron may then interact with another atom and in so doing produce another photon of X-rays of variable although reduced energy. In addition to this the X-ray photons produced by electron collisions can themselves interact with adjacent atoms and thereby produce reduced energy photons. X-ray spectra are commonly termed to be continuous, white or polychromatic (this is because there is no minimum energy). Figure 5.2 shows the form of a typical X-ray spectrum. Note: That higher energy X-rays have shorter wavelengths.

NDT20-71015 X-Rays or Bremsstrahlung

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Radiation intensity (photons per square metre)

Characteristic radiation peaks

Increasing wave length, decreasing photon energy Figure 5.2 X-ray spectrum.

The value of the minimum wavelength produced by an X-ray tube having an accelerating voltage V can be estimated using the following formula:

The minimum wavelength, 

[nanometers]

, is sometimes called the threshold wave-length.

The maximum intensity (the number of photons per square metre) in the continuous spectrum produced occurs at approximately2 ∙  . The ability of the X-rays to penetrate matter depends on their photon energy, the shorter the wavelength, the higher the photon energy, the more penetrating the radiation. The penetrating power of the X-rays can be controlled by increasing or decreasing the accelerating voltage, the greater the accelerating voltage, the more penetrating the radiation. In an X-ray set the accelerating voltage is the tube voltage. The total number of photons produced at all wavelengths is directly related to the number of high velocity electrons arriving at the target. The total number of electrons is directly proportional to the magnitude of the electric current passing through the accelerating field. This current in an X-ray set is referred to as tube current. Radiation intensity is directly proportional to tube current.


5-2


The two characteristic peaks shown in Figure 5.2 are caused by target material inner shell electrons jumping to a higher energy level, and then falling back to their equilibrium state. Characteristic radiation generally occurs at relatively low energy, long wavelength and is of no great importance in the industrial radiography of metallic components although it can cause a problem known as diffraction mottling (see the section on artefacts). As the name suggests, each element produces its own specific characteristic peaks, and measurement of these can be used to perform chemical analysis (X-ray fluoroscopy). Low energy X-rays can be diffracted by crystalline materials such as metals. In the diffraction process radiation is deflected from its original path at an angle that is determined by its wavelength and the spacing of the atoms in the crystalline material. This effect can be used to produce the mono-wavelength X-rays that are used in X-ray crystallography. 5.1

X-ray equipment In order to produce X-rays three things are required: 1 2 3

Source of electrons. Target, constructed from a suitable high melting point material. Means of accelerating electrons toward the target.

High velocity electrons cannot travel far in air, therefore the process of acceleration must take place in a high vacuum. 5.1.1

The cathode The source of electrons is called the cathode. In a conventional X-ray tube it consists of a tungsten filament heated by passing a small current through it. Heating the filament produces a cloud of loosely bound, low kinetic energy electrons in close proximity to the filament. This process is known as thermionic emission. Electrons are negatively charged and can be accelerated toward the target by making it positively charged with respect to the source of electrons. Copper focusing cup

Tungsten filament

Figure 5.3 Section through a typical X-ray tube cathode.

5.1.2

The anode (target) The anode consists of a heavy section of high conductivity copper with a small tungsten (or other high melting point high atomic number metal) insert which is called the target. The anode has a positive electrical potential with respect to the cathode. The body of the anode is always copper because it is an efficient conductor of heat. This property is necessary because approximately 95 % of the kinetic energy of the impacting electrons is converted to heat at the anode.


5-3


The target material is usually tungsten because this has a very high melting point (above 3400 °C). This reduces the chances it will be vaporised by the large amount of heat generated. Tungsten has a high atomic number and therefore a large number of electrons, this makes it a relatively efficient material for converting kinetic energy to X-ray energy which in turn helps to reduce the amount of heat produced as a proportion of the total output of energy. Sometimes the target is constructed from tantalum (melting point 3000°C) and less frequently from other refractory metals. Nearly all anodes are hooded (see Figures 5.4 and 5.5). The hood is a high conductivity copper shroud which is designed to intercept stray electrons and prevent them from hitting the tube walls. The hood has a window in the form of a beryllium insert or a thinned section of copper which permits X-rays to exit without unduly increasing inherent filtration. Inherent filtration is the term used to describe removal of X-rays from the primary beam due to absorption by the materials used in X-ray head construction. The reason that a beryllium window is used in many X-ray heads is that beryllium has a very low absorption factor and this minimises inherent filtration whilst still affording the tube walls protection from stray electrons. Anodes may be directional (Figure 5.4) or panoramic (Figure 5.5). In either case anode design is such that the effective focus size in the direction of the useful beam is much smaller than the actual focus size. This arrangement is called a line or Benson focus and it serves to maximise anode life without unduly compromising image quality. High conductivity copper Hood Electrons Cooling fins Beryllium window

Tungsten target Useful X-rays

Figure 5.4 Directional anode.


5-4


High conductivity hood

Hood

Electrons Cooling fins Beryllium window

Tungsten target Useful X-rays

Figure 5.5 Panoramic anode.

The target is generally set at an angle of about 70 to the electron beam as shown in Figures 5.4 and 5.5. This produces a small effective focus size whilst maintaining a large actual focal spot size. The large actual focus size helps to dissipate the heat generated more efficiently. Therefore higher tube currents can be sustained without the risk of damaging the target. This design feature is known as Benson or line focusing, see Figure 5.6 below. Effective focus size viewed from B = 4 x 3.8mm

Electrons

Target

Actual focus size viewed from A = 4 x 11mm

70o

Figure 5.6 Line or Benson focus.


5-5


5.1.3

X-ray tubes The cathode and anode are mounted in an evacuated glass (or in modern tubes metal-ceramic) envelope (Figure 5.7). The tube may be provided with shielding to absorb any unwanted radiation that is not already shielded out by the natural geometry of the anode. Directional type tubes produce a useful beam of radiation that is usually in the form of a cone with a dihedral angle of around 40°. X-ray tubes fitted with a panoramic anode produce a useful beam of radiation through an angle of 360° about the tube axis. Aluminium shell Ceramic insulator Anode

Cathode

Electrons

X-rays

Evacuated chamber

Ceramic insulator

Figure 5.7 Directional X-ray tube (metal-ceramic type).

The X-ray beam produced is filtered by the wall of the glass (or metal-ceramic) envelope. This reduces the useful quantity of X-rays produced, with the low energy components of the spectrum being particularly affected. Therefore it is common in glass tubes that the tube wall is ground thinner in the region of the useful beam in order to minimise the X-ray energy lost due to self-filtration. Metal-ceramic X-ray tubes (and low kilovoltage glass tubes) may have beryllium inserts (usually called windows) in order to minimise the filtration effect of the tube wall. Beryllium is used because it has a very low X-ray absorption coefficient and it is mechanically strong enough to contain the necessary vacuum. X-ray tubes are invariably mounted inside some form of tank. This is usually a metal cylinder that may be fitted with a beryllium or plastic window to minimise self-filtration of the X-rays produced. The tank contains a coolant which may be oil or some type of gas. It provides high voltage insulation and mechanical protection. In portable equipment the high voltage transformer is mounted inside the tank.


5-6


5.1.4

X-ray tube power supply In order to produce a beam of electrons from the filament in the tube it is necessary to make the anode positive with respect to the cathode. If an AC supply is connected across the tube then the beam of electrons will pass only when the anode is positive and the tube will act as a half-wave rectifier.

Applied single phase AC voltage

Resultant tube current flowing

Figure 5.8 Current flow across a half-wave self-rectified X-ray tube.

Older type portable X-ray sets were half-wave self-rectified. This produced a considerable weight-saving compared with the earlier types of constant potential unit. Most modern portable units are constant potential and use lightweight solid state rectifiers to produce what is effectively DC current. The metal-ceramic tubes used in modern equipment are able to safely withstand a greater potential difference between the anode/cathode and the tube wall. This permits the use of grounded anode type circuitry which in turn permits direct water-cooling of the anode. In an older type unit operating at say 200 kV the cathode voltage would have been minus 100 kV while the anode voltage would have been plus 100 kV, giving a maximum potential difference between the electrodes and the glass tube wall of 100 kV. With modern grounded anode circuitry it is safe to hold the cathode at minus 200 kV with the anode at zero volts to produce the same 200 kV potential difference. An anode held at zero volts can be safely cooled by water. Water is a very efficient coolant and direct water-cooling of the anode permits operation at greatly increased tube currents. For example, the maximum tube current for an older type 200kV oil-cooled head was typically 5mA self-rectified. With modern portable equipment maximum constant potential tube currents of 15 or 30 mA are not unusual for a 200kV head. Older type constant potential industrial X-ray units were extremely heavy, bulky and suitable for use only in fixed installations. Much of the weight and bulk came from the rectification circuitry used, the so called Greinacher Circuit and the large external oil-cooling system necessary to dissipate the large amount of heat generated.


5-7


Single phase AC voltage

Full wave rectified AC voltage

Greinacher circuit

Figure 5.9 Greinacher circuit voltage.

Number of protons

The total quantity of X-rays produced by an X-ray unit is directly proportional to the area under the line showing voltage against time. Thus for the same tube current and peak voltage, an X-ray tube using a smoothed and fully rectified supply (ie a constant potential unit) will produce more X-rays than a selfrectified tube. In fact the output of X-rays is more than doubled for the same tube current. In a self-rectified unit the tube voltage varies from zero to the peak voltage and back again with each cycle. In a constant potential unit the tube voltage is close to constant. Thus, looking at the spectrum of X-rays produced, a self-rectified unit produces proportionally more low energy radiation than a similar constant potential unit.

Self-rectified Constant potential Wavelength

Figure 5.10 X-ray spectra of constant potential and self-rectified tubes.


5-8


5.1.5

X-ray tube controls The radiation produced by the X-ray tube can be varied in quantity and penetrating power (or quality) by controlling the electrical supplies to the tube. Penetrating power or radiation quality (kV) The penetrating power of X-rays depends on the magnitude of the accelerating voltage which is applied between the cathode and the anode. The higher the voltage, the higher the kinetic energy of the accelerated electrons, the higher the photon energy of X-rays produced. The higher the photon energy, the shorter the wavelength, the greater the penetrating power. Thus the penetrating power or quality of X-rays is controlled by the tube voltage. Conventional X-ray tubes, as used in industrial radiography, are capable of being operated in the range from below 50 to 450 kV. If greater penetrating power is required high energy X-ray sources such as betatrons, linear accelerators or Van der Graaf generators can be used to provide X-ray energies of up to 30 or even 40 MeV. Some recognised codes and standards such as ASME BPVC V/2 and EN ISO 17636-1 relate the maximum kilovoltage which may be used to the material thickness which is to be examined. Table 1 gives the approximate limiting maximum economically penetrable thicknesses of steel for various kilovoltages. The figures given are typical for film radiography using lead intensifying screens and portable self-rectified equipment. Constant potential units can be used economically on greater thicknesses than can self-rectify units.

Figure 5.11 EN ISO 17636-1 maximum permissible X-ray tube voltage for various materials.


5-9


Occasionally penetrating radiation is referred to as being either hard or soft. These terms are relative, but hard radiation is produced by high tube voltages (above 150kV) whilst soft radiation is produced at lower tube voltage. Table 5.1 Approximate kilovoltages.

Kilovolts 120 160 200 250 300

penetrating

power

in

mm

of

steel

for

various

Penetrating power, mm of steel 6 20 30 45 60

Quantity of radiation (mA) The quantity of radiation produced by the X-ray tube per unit time depends on the number of electrons released by the cathode filament. The number of electrons per second reaching the anode multiplied by the charge on the electron is equal to the tube current. The tube current is not controlled directly, it is increased or decreased by controlling the size of the heating current supplied to the cathode filament - the higher the heating current, the hotter the filament, the greater the thermionic emission of electrons and hence the greater the tube current. Tube current will also be increased for the same heating current if the tube voltage is increased. This is because higher voltages can draw more electrons from the filament even though the filament temperature does not change. So if the tube voltage is altered it will be necessary to adjust the heating current if the same value of tube current is to be maintained. Too high a tube current would cause damage to the anode due to overheating; therefore X-ray equipment always incorporates a safety cut-out switch to prevent the use of a too high value of tube current. The total quantity of radiation produced by the X-ray set is directly proportional to the product of the exposure time (ie the time for which the X-ray tube is energised) and the tube current; therefore X-ray exposures are usually given in milliampere minutes (mA.min) at a given tube voltage. The standard controls on the X-ray set are: 

Voltage control: This alters the tube voltage (kV) by varying the low voltage supply to the high voltage transformer. Note: That high voltage is not generated in the control panel, this minimises the hazard to personnel.



Milliampere control: An ammeter incorporated into the control panel measures (albeit indirectly) the current flowing across the tube. This is proportional to the number of electrons flowing from the cathode to the anode per unit time. In order to increase the supply of electrons the heating current to the filament is increased using the milliampere control. Note: That the ammeter measures the current flowing between the anode and the cathode, not the current flowing in the filament (ie the heating current).



Timer: Since the quantity of X-rays produced is proportional to the length of time during which the tube is energised it is convenient to incorporate a time-switch into the control panel of the X-ray set which automatically terminates the exposure at a preset time.


5-10


As explained above it is convenient to refer to X-ray exposures in terms of milliampere minutes. For example an exposure which produces an acceptable radiograph may have been determined to be, 36mAmins at 200kV. If this was the case then any of the following exposures should give an identical acceptable result: a b c

9mA for 4min at 200kV. 18mA for 2min at 200kV. 2mA for 18min at 200kV.

Reciprocity law

This is because the amount of radiation produced is the same in each case. Obviously it would be desirable to use a high value of mA, in order to reduce the exposure time, but as explained above the use of high tube currents can severely damage the anode of the X-ray tube and thus reduce its service life. Therefore it is usual to operate at a value of mA which is well within the tube’s specified capabilities. The reciprocal relationship between time and tube current is sometimes referred to as the reciprocity law or the Bunsen Roscoe reciprocity law. 5.1.6

High energy X-ray sources Betatrons Betatrons are used to produce ultra-hard extremely penetrating radiation with photon energies in the range 1 - 30MeV. The efficiency with which the kinetic energy of the accelerated electrons is converted to X-rays is much better at high voltages than at those experienced in conventional X-ray tubes. Consequently betatrons usually benefit from quite small focal spots. In betatrons electrons are accelerated in a spiral path of perhaps 1,000,000 revolutions by means of alternating magnetic fields before being deflected towards the target. The radiation produced by betatrons can penetrate 300 mm or more of steel. They are primarily used for the radiography of castings or large section welds in fixed installations but portable units are available. These are sometimes used on site for the inspection of reinforcing bars in heavy concrete sections. Up to around 10M eV betatrons are usually preferred to linear accelerators because they are more compact and less expensive to manufacture. Linear accelerators Linear accelerators (often called linacs) accelerate electrons to very high velocities along a straight path by means of an electromagnetic waveform generated by a device called a magnetotron. The particle velocities are similar to those achieved in betatrons but a much higher output of radiation is achievable. For radiation energies above 10MeV linear accelerators are generally the preferred solution. Van der Graaf generators Van der Graaf generators generate a high voltage charge of static electricity by mechanical means - friction. This can be used to accelerate electrons for X-ray production. Van der Graaf generators can produce short intense pulses of X-ray energy so have therefore found some application in the field of ballistic radiography.


5-11


5.1.7

Special types of X-ray unit Microfocus X-ray sources Standard X-ray equipment has an effective focus size usually in the range 0.8 4 mm. This is small enough to provide adequate image quality for most standard techniques. Micro-focus X-ray equipment may have an effective focus size as small as 0.1mm. Using such a small focus size geometric enlargement techniques is possible whilst still producing an adequately sharp image. Focus

Object

Image

Un-sharp image (standard 4mm focus)

Sharp image (0.2mm micro-focus)

Umbra Penumbra Figure 5.12 Focal spot size effect on penumbra.

Rod anode X-ray tubes

Aluminium tube

Cathode

X-rays

In a rod anode tube the target is at the end of a copper or aluminium tube which is usually less than 20mm outside diameter and may be up to a metre long. The target is invariably of the panoramic variety. Grounded anode circuitry is essential for this type of tube. The anode can be positioned inside small diameter pipes in order to carry out panoramic radiography of girth welds; it can also be positioned in many other otherwise inaccessible locations. Rod anode tubes are most often used in aerospace applications.

Electron

Anode X-rays

Evacuated chamber Ceramic insulator

Aluminium shell

Figure 5.13 Rod anode X-ray tube.


5-12


Rotating anode X-ray equipment In medical radiography a very large tube current is generally desirable as this permits a very short exposure time which in turn helps to eliminate or reduce unsharpness caused by relative movement during exposure. To maximise tube current some medical equipment is fitted with a rotating anode, where the anode rotates at high speed and the focus area of the target is therefore constantly changing. Each section of the tungsten target is in use for a short time followed by a slightly longer period of resting. This helps to prevent overheating so the tube current can be greatly increased.


5-13


Section 6 Gamma Rays

6

Gamma Rays Gamma () ray is the term applied to the electromagnetic radiation which is sometimes produced when the atomic nuclei of a radioactive isotope disintegrate in the process known as atomic fission. Alpha () and beta () particles may also be produced during the disintegration process; in fact gamma emission is always a by-product of alpha or beta emission. Of the three main types of radiation produced by fission alpha is by far the most hazardous to health; alpha and beta radiation must be taken into consideration when assessing safety. Except as a health hazard, alpha and beta particle radiation have no significance for industrial radiography since they are easily absorbed by very thin materials. The disintegration process is fixed for each radioactive isotope and as a result the gamma ray energies produced are also fixed. 60 27

60 Co 28 Ni 

   0.31 MeV



1.17 MeV

 1.33 MeV

The spectra produced are line or discrete spectra as opposed to the continuous spectra produced by X-ray equipment. Table 6.1 lists the principal gamma emissions for various commonly used isotopes. Figure 6.1 shows the line spectrum for Iridium 192. 6.1

Alpha and beta emission

6.1.1

Alpha particles Alpha particles are emitted during the decay of heavy nuclides such as uranium (U) 238 and plutonium (Pu) 239. An alpha particle consists of 2 protons and 2 neutrons – basically a helium (He) nucleus, emitted from the nucleus at very high velocity. For example: 239 94

4 Pu  235 92 U  2 He

Thus in alpha emission there is a loss of 4amu from the nucleus and a reduction in atomic number of 2 (see the example above). Alpha particle radiation cannot penetrate more than a thin sheet of paper or a few centimetres of air, it is, however, very strongly ionising. The great danger to health with alpha emitters is that they may be ingested – radioactive contamination. Once within the human body they will in most cases cause cancer. 6.1.2

Beta particles Beta particles may be emitted during radioactive decay. A beta particle consists of a very high velocity electron emitted from the nucleus of a radioactive atom when a neutron converts to a proton. Note: That although the beta particle is an electron it has very much higher kinetic energy than a free electron which has resulted from an ionisation event. For example: 14 6

C 147 N  E 

NDT20-71015 Gamma Rays

6-1


Thus in beta emission there is no loss from the atomic mass number whilst the atomic number increases by 1 (see the example above). Beta radiation is more penetrating than alpha. It can penetrate the outer layers of the skin and lead to fatal skin burns. The damage caused is very similar to sunburn, but much more severe. Many of the early victims of the Chernobyl disaster died as a result of skin burns caused by exposure to high intensities of beta radiation. If beta emitters are ingested they will often lead to cancer. 6.2

Sealed Sources The first gamma ray emitting radioisotopes to be used in industrial radiography were naturally occurring radioactive materials such as radium. Such sources were not sealed and therefore there was a danger of exposure to alpha () and beta () particles, both of which are extremely damaging to human tissue. Coupled with this was the even greater hazard of radioactive contamination by which radioactive materials might find their way into the human body. All gamma sources in use today are man-made. They are manufactured by neutron bombardment of non-radioactive raw materials in the core of a small nuclear reactor. The sources in use are all beta emitters, gamma rays being produced as a by-product of beta emission. In order to prevent beta emission or contamination hazard the sources used in industrial radiography are invariably sealed sources. The fissile material is encapsulated in a high integrity titanium or stainless steel shell. Beta radiation is not capable of penetrating the walls of the capsule, and the capsule further precludes any possible contamination hazard so long as it remains intact.

Note: That the Iridium 192 spectrum includes several other less significant gamma emissions. The emissions shown account for more than 98% of all gamma radiation produced.

Figure 6.1 Iridium 192 - principal gamma emissions.


6-2


Table 6.1 Gamma emissions for commonly used isotopes.

Equivalent X-ray kilovoltage, kV 400 - 600

Penetrating power in mm of steel 20 - 100

Isotope Iridium (Ir) 192

Half-life 74.4 days

Principal emissions, MeV 0.31, 0.47, 0.60

Cobalt (Co) 60 Thulium (Tm) 170

5.3 years

1.17, 1.33

2000 - 2500

40 - 200

127 days

0.052, 0.084

80

up to 5

Ytterbium (Yb) 169

32 days

0.17, 0.20

145

1 - 15

Selenium (Se) 75

118.5 days

0.121, 0.136, 0.265, 0.28, 0.401

320 - 450

10 - 40

Isotope Glass fill material Titanium capsule High integrity weld Connection device (stainless steel)

Figure 6.2 Sealed source.

Figure 6.2 shows the typical encapsulation arrangement for iridium 192 and cobalt 60. Some isotopes such as caesium 137 are double encapsulated. In the case of caesium 137 this is because it is in the form of caesium chloride which is highly corrosive and highly water soluble (but this is still an improvement on caesium metal which causes an explosion on contact with water). 6.3

Penetrating power of gamma radiation The penetrating power is fixed for each isotope because the spectrum of gamma radiation emitted is fixed. If a material thickness is too great to produce a radiograph using, say, iridium 192 then an isotope which produces higher energy gamma radiation, such as cobalt 60, must be used.

6.4

Equivalent energy of isotopes Due to fact that linear spectra of isotope usually consists of more than few discrete energies it is not easy to determine which of these is predominant. In order to allow comparing gamma sources with X-ray the term equivalent energy was introduced sometime the equivalent energy is deducted from envelope of linear spectra of given isotope. In such case it depends how much simplified spectrum is used (Iridium has three of four main energy lines which are usually considered, but in total there are 24 known lines) different approach is to compare effect of radiation to detector. This approach reflects proved fact that isotopes providing less contrast due to higher energy (compared to X-ray) – see Table 6.1.


6-3


6.5

Radiation intensity The amount of gamma radiation, the number of photons, produced by an isotope is controlled by the number of disintegrations (atomic fissions) per unit time. The source strength of an isotope is usually expressed in curies (Ci) or Becquerels (Bq). Source strength may also be referred to as source activity. 1 Ci = 3.7 x 1010 disintegrations per second 1 Bq = 1 disintegration per second The Becquerel, which is the SI unit of radioactivity, is a very small unit in terms of what is required for industrial radiography. The Curie is therefore generally preferred. If the Becquerel is used at all then it is usually in the form of gigabecquerels (GBq). One gigabecquerel is equal to one thousand million (109) becquerels. One curie is equal to 37 gigabecquerels (37GBq). In the majority of cases gamma ray exposures are expressed in curie-hours, curie-minutes or curie-seconds; this in each case being the product of source strength measured in curies multiplied by exposure time measured in hours, minutes or seconds. Example: A steel section 50 mm thickness requires an exposure of 700 curie-minutes using iridium 192 with a source to film distance of 1 metre using Kodak CX film and lead intensifying screens. All other factors being equal the exposure time would therefore be either: a

1 hour 10 minutes with source strength of 10 curies.

b

20 minutes with source strength of 35 curies.

c

7 minutes with source strength of 100 curies.

Reciprocity law

Gamma rays are produced by a disintegration process. Atoms having unstable nuclei decay with a fixed probability to form other atoms having stable nuclei. Therefore the source strength of the radioactive isotope will reduce with time. The probability decay for a large number of unstable atoms is fixed and proportional to the number of unstable atoms present. This means that the strength of a radioactive source will always reduce exponentially: ie the strength of a given source will reduce by 50 % in a fixed time. This fixed time is referred to as half-life. The half-life of various commonly encountered isotopes are given in Table 6.1. If the half-life of an isotope is known then the source activity at a given time can be calculated if the source activity had previously been measured. Suppose that an isotope having a half-life h, had an activity S0, at time t = 0. Then at time t, the source strength or activity St, can be calculated using: St = S0 2-(t/h) Alternatively the activity of a source can be estimated using a decay chart. Figure 6.3 shows the decay chart for iridium 192.


6-4


Figure 6.3 Iridium 192 decay.

6.6

Radioactive isotope containers for industrial radiography Radioactive isotopes emit gamma rays continuously; the decay process cannot be switched off or in any way slowed down. Gamma radiation is extremely harmful to human body tissues so radioactive isotopes must be shielded when not in use. The shielding materials used in isotope containers are always dense materials such as lead, tungsten or (more commonly) depleted uranium. Most modern containers use depleted uranium shielding because uranium is an extremely efficient absorber of gamma radiation. Uranium shielded isotope containers are much lighter and more portable than their lead shielded counterparts. A uranium shielded container having a weight of about 20kg can safely store 100 Ci of iridium 192. A lead shielded container of the same weight would be capable of safely containing only 20 Ci of iridium 192. Radioactive isotope containers are designed to fulfil two important functions: 1

Contain the radioactive isotope and reduce the emitted intensity of radiation to a level which allows for safe transportation and storage.

2

Allow the radioactive isotope to be safely exposed in order that it may be used for radiography.

In addition, radioactive isotope containers have to be capable of withstanding possible accidents involving impact or fire. All modern isotope containers are designed to be operated by cable (see Figure 6.6). They are of two basic types, (Figures 6.4 and 6.5). Of the two types depicted the S tube type is intrinsically safer but around 30% heavier than the equivalent shutter type. Older types of isotope container did not provide for remote operation.


6-5


Sealed source

Locking device

Connector for delivery tube

Connector for wind-out

Fireproof packing material

Depleted uranium or tungsten shielding

Figure 6.4 S tube type radioactive source container.

Locked position

Unlocked position

Figure 6.5 Shutter type radioactive source container.

Projection tube

Isotope container

Wind-out

Figure 6.6 Remote control isotope delivery system.


6-6


Figure 6.7 Sealed source with flexible cable (pigtail) attached.

6.7

Comparison of X- and gamma rays

6.7.1

Energy and output of radiation X-ray equipment produces a continuous range of photon energies up to a threshold level dependent upon the tube voltage setting. The threshold photon energy level can be adjusted from 50keV or less up to a maximum (for high energy equipment) of perhaps 30MeV. The photon energy of gamma ray sources is fixed. The output of radiation per unit time is variable for X-ray equipment up to the maximum mA rating of the tube. The output of radiation from a radioactive isotope is fixed by the source activity. The output of radiation produced by Xray equipment is generally much greater than that produced by radioactive isotopes. The penetrating power of ionising radiation is controlled by its maximum photon energy and the photon energy distribution. Table 6.2 gives an indication of the maximum steel thickness that can practically be radiographed using conventional X-ray equipment and the commonly encountered isotopes. The penetrating power of X-rays produce by self-rectified equipment is less than that of X-rays produced by constant potential equipment operating at the same tube voltage. This is because the constant potential equipment produces a larger proportion of high energy radiation than does the self-rectified.


6-7


Table 6.2 Useful thickness range for various sources of radiation

Source of radiation 100 kV (peak) X-ray 150 kV (peak) self200 kV (peak) rectified 300 kV (peak) 100 kV X-ray 150 kV constant 200 kV potential 300 kV Thulium 170 Selenium 75 Gamma ray Ytterbium 169 Iridium 192 Cobalt 60

Useful thickness range/mm of steel, mm Maximum 8 Maximum 20 Maximum 30 Maximum 60 Maximum 10 Maximum 32 Maximum 45 Maximum 100 Maximum 5 10 - 40 5 - 15 20 - 100 40 - 200

Note: Steel sections of 500 or 600mm can be radiographed using X-rays generated by linear accelerators or betatrons. 6.7.2

Radiographic contrast Low energy radiation is more easily absorbed than high energy radiation. Therefore low energy radiation will show a bigger change in radiation intensity for the same change of penetrated section thickness than will high energy radiation. Thus radiographs made with low energy radiation will usually show better contrast than those made using high energy radiation. The contrast of X-radiographs is generally better than gamma radiographs since it is possible to optimise the X-ray energy for the thickness of the material which is to be examined; in so doing obtaining the best possible contrast. With a gamma ray source the radiation energy is fixed and is optimum for a narrow range of thickness only. Contrast is better for X-rays of the same maximum radiation energy than it is for gamma rays because X-ray tubes produce a continuous range of energies as opposed to the line spectrum which is obtained from a gamma ray source.

6.7.3

Focal spot size versus source size A radiograph produced using small effective source size will usually be of higher quality than one produced with a larger effective source size. The average focal spot size an X-ray tube is similar to the average physical size of the gamma ray sources which are commonly used. Most X-ray tubes have a fixed effective focal spot of between 1 and 4mm. With some X-ray equipment the focal spot size can be varied. Microfocus X-ray tubes may have an effective focal spot of less than 0.1mm. The size of the focal spot in an X-ray tube tends to be larger for the higher maximum kilovoltage tubes. This is due to the need to dissipate the increased amount of heat generated at high kV. The practical source size for a radioactive isotope is determined by the maximum economically achievable specific activity. Specific activity is usually expressed in curies (or becquerels) per gram. Table 6.3 below gives typical practical achievable maximum specific activity for 4 common isotopes.


6-8


Table 6.3 Specific activity for common radioisotopes

Isotope Cobalt 60 Iridium 192 Caesium 137 Thulium 170

Practically achievable maximum specific activity, curies per gram 50

Density, g/cm3 8.9

Maximum practically achievable activity for 3mm diameter, 3mm long cylindrical pellet, curies 10

350

22.4

166

25

3.5

1,000

4

(1)

(2)

2 85

Note: 1 Density is for compressed caesium chloride (CsCl). 2 Density is for thulium oxide (Tm2O3). Note: That the maximum activity of a gamma ray source is limited by its physical size. The most useful isotopes are those which have a high value of practically achievable specific activity. In an iridium 192 source at the maximum achievable activity, about 2.5 atoms per 100 million are radioactive. In a cobalt 60 source the figure is only about 1 atom in every 10,000 million. The output of radiation from a typical X-ray machine is much greater than from a typical gamma source. This means that in X-radiography the use of long focal to film distances is more economically feasible than in gamma radiography. Thus, even though the focus is similar in physical size when compared with the average gamma source, it is generally the case that geometric unsharpness is better for X-ray techniques than for gamma. 6.7.4

Exposure time (film radiography) An exposure time of between 0.5 and 5 minutes is usual for X-ray radiography. An old conventional self-rectified X-ray set operating at maximum kilovoltage and tube current will generally be capable of continuous use with an exposure time of up to 5 minutes followed by a rest period between successive exposures of around 1 or 2 minutes. If the exposure time is extended beyond 5 minutes then overheating will generally occur if the rest period is not considerably extended. Modern constant potential equipment intended for fixed installation usage will usually be capable of continuous operation at its maximum output rating. However, even with such equipment, exposure times exceeding 10 minutes will generally be avoided. The exposure time for gamma radiography tends to be longer. This is because the output of radiation (in photons per second) is generally much less. Gamma ray exposure times are usually in the range from about 30 seconds to 1 hour, but exposure times exceeding 24 hours are not unheard of. The required exposure time for a gamma ray source increases as the source activity reduces with time.

6.7.5

Power supply X-ray sets require power from a mains supply or mobile generator. Usually a 4.5kW generator will provide sufficient power to operate a 300kV self-rectified set. Gamma radiography can in general be carried out without the need for a power supply.


6-9


6.7.6

Physical size and weight An iridium 192 isotope with a source activity of up to 100 curies can safely be stored in a container weighing 15-20kg which has outside dimensions of approximately 200 x 400 x 100mm. Such isotopes are useful for the radiography of steel sections of up to 75mm thick. Gamma ray sources can be used to make exposures in situations where access is extremely limited. A typical self-rectified 300kV rated X-ray set (which is useful for the radiography of steel sections of up to 60mm thickness) is on the other hand considerably less portable and less manoeuvrable. A typical 300kV self-rectified tube head could weigh 55kg and measure 300 x 300 x 750mm while the associated control panel might weigh as much as 30 kg and measure 450 x 350 x 250mm. Low kilovoltage equipment offers improved portability and manoeuvrability but this has to be offset against the reduced penetrating power.

6.7.7

Equipment cost The initial cost of X-ray equipment for site work is about 2-5 times that of a portable gamma ray container. The cost of maintenance and repair is greater for X-ray equipment, due to the nature of the electrical equipment involved and because X-ray equipment is less rugged and therefore more prone to damage in site conditions. Gamma ray sources have to be replaced on a regular basis due to radioactive decay. This can become costly if the source is not used regularly. Gamma ray sources have, by law in most countries, to be stored in very secure conditions. This factor also adds to costs when compared to X-ray equipment. X-ray exposures tend to be shorter so there can be a cost saving with X-ray equipment if the setting-up time between successive exposures is minimised. Overall gamma radiography tends to be the most cost effective solution for construction site work but X-radiography may provide the cheapest option where are large number of similar radiographs are required (such as may be the case in pipeline or mass production environments).


6-10


Section 7 Methods of Producing a Radiographic Image

7

Methods of Producing a Radiographic Image

7.1

Radiographic film Radiographic film is essentially the same as that used in photography in that it consists of a suspension of silver halide grains in a gelatine binder on an acetate or polyester base. Radiographic film, however, differs from photographic film in that the:  



Acetate or polyester base material is considerably thicker than photographic film. Emulsion is applied to both sides of the film. This effectively doubles the film density (i.e. degree of darkness) for the same exposure to radiation and thereby doubles the film speed. Emulsion tends to be thicker (usually around 0.025mm) than or photographic films, in order to further increase the film speed.

Two types of radiographic film are used for industrial radiography: Direct type film: Where the principal cause of image formation is the ionising radiation itself. This may be coupled with the effect of secondary electrons emitted from metallic foil intensifying screens. Screen type film: Where the principal cause of image formation is light emitted from fluorescent image intensifying screens under the action of ionising radiation. Super-coat Emulsion 0.2mm

Subbing Base Subbing Emulsion Super-coat

Figure 7.1 Cross-section through a radiographic film.

Some radiographic film will produce good results either as a direct type or as a screen type film. The film emulsion in screen type films usually has a matt finish so as to avoid reflecting the light produced by fluorescence. All standardisation systems are classifying radiographic films into several categories on basis of their gradient (respectively grain size). EN ISO 11699-1 recognised in total 6 film classes from fine grain category C1 to coarse grain category C6. In order to allow some standardisation of procedures in radiography, it is common requirement that manufacturer shall classify the film in one of categories (see Table 7.1).

NDT20-71015 Methods of Producing a Radiographic Image

7-1


Table 7.1 Classification of radiographic films Classification

Manufacturer designation and film factor CF

EN ISO 11699-1

ASTM E 1815

C1 C2 C3 C4 C5 C6

Special Class 1 Class 2 Class 3

AGFA

CF

KODAK

CF

FUJI

CF

FOMA

CF

D2 D3 D4 D5 D7 D8

9.0 4.2 2.6 1.6 1.0 0.7

DR 50 M 100 MX 125 T 200 AA 400 CX

7.2 4.2 2.8 1.7 1 0.7

IX 25 IX 50 --IX 80 IX 100 IX 150

6.5 3.3

R2 R3 R4 R5 R7 R8

9.0 4.2 2.6 1.6 1.0 0.7

1.6 1.0 0.6

Note: Not all film categories are suitable for given purpose. Therefore for example standard EN ISO 17636-1 lists only 3 classes (C3 to C5). 7.2

Film speed Film gradient is related to size of sensitive grains dispersed in emulsion. Films with bigger grains are usually considered to be more sensitive – it means that such film turn black faster while exposed to radiation than fine grain film. Coarse grained film is more sensitive to radiation because it requires a shorter exposure time than fine grain film because each grain of silver halide needs only to receive as few a single photon of radiation or single secondary electron in order to become sensitised. When a sensitised grain contacts the developer solution the entire grain, regardless of its size, is converted to image forming metallic silver. Large grains of silver will block out more light than small grains so a coarse grained film will appear darker after processing than will a fine grained film even though the exposure conditions were exactly the same. On the other hand fine grain film is offering better resolution of radiograph and also better contrast. From this we can say that fine grain film = slow film and coarse grain film = fast film. The designation “fast” and “slow” is typical for US standards and codes while EN ISO rather speaks about film factor CF.

7.3

Film factor CF Film factor is another way how to express the film speed. Film factor is related to reference level and express how many times film faster or slower is. It is non-written agreement nowadays that AGFA D7 is reference level (CF = 1) and other factors are related to it. Because film AGFA D7 is classified as film class C5, all other films classified in this class are said to has same film factor CF = 1. There may be small variation in speed of particular forms in given film class due to fact that class represents some band of film speed values. Advantage of film factor (against film speed) is that film factor is represented by numerical value which can be used for recalculation of exposure time when it is necessary to interchange given film by another.


7-2


7.3.1

Latent image formation When film is exposed to X-rays, gamma rays, or light, an invisible change called a latent image is produced in the film emulsion. The areas exposed become dark when the film is immersed in a developing solution, the degree of darkening depending on the amount of exposure. Photographic emulsion consists of myriads of tiny crystals of silver halide (usually silver bromide with a small quantity of silver iodide) dispersed in gelatine. Each tiny crystal responds as an individual unit during exposure to radiation and subsequent development. A latent image can be defined as that radiation induced change in a silver halide crystal that renders the crystal susceptible to the chemical action of the developer. Photography utilising film emulsion similar to that in use today has been with us since around 1839 but the mechanisms involved in latent image formation remained a mystery until 1938 when the Gurney-Mott theory was first put forward. Although this theory is now generally accepted there remain areas of speculation. Formation of a latent image involves a very subtle change in the silver halide grain. It is known to involve the absorption of only one or a few photons of radiation. Because of the small amount of energy involved it is obvious that only a few atoms, out of the ten thousand million or so atoms in a typical silver halide grain, can actually be affected. To date it has proved impossible to detect either the physical or the chemical nature of the tiny changes involved. Against this, however, much can be deduced about what the physical nature of these changes must be. For one thing we know that the substance which forms the radiographic image must be metallic silver. We also know that the latent image is localised at certain discrete sites within the silver halide grain. The evidence for this is shown in Figure 7.2 an electron micrograph of a section of film emulsion that has been exposed to light followed by brief contact with developer. Note how tiny amounts of silver (the dark areas) have appeared (the dark areas) within each grain of silver halide. Further it is known that prolonged exposure to light will darken the film emulsion even without development. Therefore the mechanism of latent image formation will by itself cause the release of silver from a silver halide grain under extreme conditions (see Figure 7.3).

Figure 7.2 Electron micrograph of exposed, partially developed, partially fixed grains of silver halide, showing initiation of development at localised sites in the grains (1µ = 1micron = 0.001mm).


7-3


Figure 7.3 Electron micrograph of the release of silver in a grain of silver halide caused by very intense exposure to light.

Based on such evidence it is fairly safe to assume the mechanism of latent image formation releases a few atoms of silver within each exposed silver halide grain. The chemicals contained in the developer then preferentially attack such exposed grains. An excellent explanation of the Gurney-Mott theory Radiography in Modern Industry on the Kodak website. 7.3.2

can

be found

in

Film cassettes Radiographic film is highly sensitive to light, in particular to light at the blue end of the spectrum. It has to be protected from exposure to light (except for the light from darkroom safe lamps) at all times up to when the fixing process has commenced. Prior to use the film is stored in light proof boxes. In order that the film can be used for radiography the film has first to be inserted into a suitable light proof container. Such containers are called cassettes. Film cassettes are of two types: rigid and flexible. Film cassettes serve three important functions: firstly they protect the film from unwanted exposure to light, secondly they help to maintain good film-screen contact and thirdly they protect the film against environmental or handling damage. Film cassettes must be manufactured in such a way that they do not produce any unwanted image on the radiograph. Some radiographic film is available pre-packed in a protective light proof envelope complete with lead intensifying screens. For the most part, flexible cassettes manufactured from opaque PVC or other plastic are used because they are cheap, durable and versatile. Film cassettes must be handled with care, as they are particularly easy to rupture during loading and unloading of the film. Cassettes which leak light can add considerably to the cost of radiography if they lead to a radiograph having to be retaken. Therefore it is good practice to inspect cassettes prior to use. Leaky cassettes can often be satisfactorily repaired using opaque adhesive tape.

7.3.3

Intensifying screens In industrial radiography intensifying screens of one form or another tend to be used for most exposures. An intensifying screen amplifies the effect that the primary radiation beam has upon the radiographic film emulsion, thus shortening the required exposure time.


7-4


Metallic foil intensifying screens At first it may seem to be a little paradoxical that metallic (nearly always lead but occasionally copper, steel or tantalum) foil screens can produce an intensifying effect. All metals are good absorbers of ionising radiation so one would naturally expect that the film density would be reduced rather than increased. In practice however, lead or copper foil screens brought into close contact with direct type radiographic film reduce the required exposure for radiation energies in excess of 120keV by a factor of about two. The reason for this is that under the action of ionising radiation of energy 120keV or greater metals produce secondary electrons which have kinetic energy sufficient to cause the sensitisation of the grains of silver halide which they strike. Metallic foil screens further add to the quality of the radiograph by filtering out a large proportion of the scattered radiation which is of lower energy (and therefore more easily absorbed) than the primary beam. For most purposes lead foil screens of thickness 0.03 to 0.125 mm are used but thicker screens are used for high energy radiography. Copper screens tend to be used only for extremely high energy techniques (above 1MeV). The lead screens found in pre-packed film are only a few microns thick, they produce a strong intensifying effect but have a much reduced effect on the scattered radiation as compared with standard re-useable lead screens. Pre-packed film is available either in individual disposable cassettes or as rollpack where a long narrow length of film is supplied complete with lead screens in a protective light proof sheath. Rollpack film can be cut to any desired length. The cut ends have to be light sealed with suitable adhesive tape. Rollpack is commonly used on pipelines in conjunction with the panoramic technique. EN ISO 17636-1 specifies metallic screens of lead, copper, steel and tantalum and the specified thickness range and screen material change for different X-ray tube voltages and different isotopes. Salt screens Salt screens consist of a layer of calcium tungstate (or other fluorescent material), attached using a suitable binding material, to a sheet of cardboard. While salt screens can produce a dramatic reduction in exposure time when used with screen type film they are seldom used in industrial radiography because they produce an image of inferior quality, are expensive and very easily damaged. Salt screens produce an image intensifying effect by fluorescing, usually the blue part of the spectrum, under the action of ionising radiation. They are capable of cutting the exposure time required by a factor of up to 500. Fluorometallic screens These screens, which attempt to combine the advantages of lead screens with those of salt screens, are occasionally used in industrial radiography when there are strong financial pressures for a reduction in exposure time. One such application is on offshore pipe laying barges. They are even more expensive than salt screens at around £70 for a pair of 10 x 40cm screens and they are just as easily damaged. They do not provide quite the same reduction in exposure time as do salt screens but the image quality is considerably improved (although still inferior to that produced using lead screens). Fluorometallic screens consist of a cardboard backing material with a layer of lead foil attached, a layer of calcium tungstate or other fluorescent crystalline material suspended in a suitable binding material.


7-5


7.3.4

Film processing Radiographic film forms a latent image during exposure to ionising radiation, light or secondary electrons. By a process which is not fully understood, silver halide grains become sensitised during exposure (see Section 7.3.1). To make the latent image formed by the sensitised grains visible it is necessary to chemically process the film. Films can be processed either manually or automatically but the chemical processes involved are the same. Radiographic film must not be exposed to light except that from darkroom safe-lamps; even this exposure must be minimised as prolonged exposure can result in film fogging. Extreme care must be exercised during film processing because the wet film emulsion is extremely fragile.

Velcro fastening Lead screen

Black PVC cassette

Film

Cardboard backing of screen Velcro fastening Figure 7.4 Combination Film / screens / cassette.

Development The first stage in film processing is development. During this stage a reducing agent such as hydroquinone or metol reduces the sensitised silver halide grains in the film emulsion to metallic silver. Development, whether manual or automatic, must be carried out within the temperature range recommended by the developer manufacturer otherwise image quality will be severely impaired. Developers for manual processing are usually designed for use at 20°C, for automatic processing this will usually be increased to around 27°C. Films should always be developed for the optimum processing time of about five minutes for manual and fewer than two minutes for automatic development. Film developed for a time not within the developer manufacturer’s recommendations will have impaired image quality. It is recommended to agitated film thoroughly in the first 20-30 seconds of development when the reaction with the developer is rapid and at regular intervals (usually 10 seconds per minute) throughout the remainder of allotted development time. Failure to properly agitate the film will result in a streaky radiograph and inferior image contrast. The developer solution undergoes chemical changes during film processing and it must be replenished regularly to maintain its effectiveness. Exposure to air must be minimised because developer is readily oxidised. Contamination of the developer with foreign material, especially metal particles, is likely to lead to unwanted images being produced on the film.


7-6


Stop bath The stop bath serves two purposes: (1) Curtail the action of the developer and (2) Protects the fixer by reducing carry-over of developer solution. It is not essential to use a stop bath but it is desirable because it will considerably extend the life of the fixer. It will also help to avoid possible film artefacts, dichroic fogging in particular. Two types of stop bath may be used, an acid or running water. In either case the stop bath must be maintained at a temperature which is comparable to that of the developer otherwise reticulation may result. The developer operates in an alkaline buffer solution. Acid stop baths neutralise the alkalinity of the developer and stop the development process almost immediately. It is normal to allow the film to remain in such a stop bath for 10-30 seconds. Running water stop baths quickly dilute the developer solution thus rapidly slowing the development reaction and minimising the damage to the fixer. It is normal to allow a time of 2-3 minutes when using running water stop baths. Fixing and hardening The chemicals which are used to fix the image and harden the emulsion are normally combined in a single chemical bath. Both types of chemical have to be protected by an acid buffer solution. In acid solution the active ingredient in the fixer will dissolve only silver halide from the film; if the solution becomes alkaline it will, in addition, begin to dissolve any metallic silver present. The silver halides which remain intact in the film emulsion after development must be removed to preserve the image which has been formed and in order that it can be viewed using transmitted light. Any silver halide which remains in the emulsion of a fully processed film will quickly deteriorate on contact with air under the influence of light to form brown stains which severely degrade the quality of the image. The process of removing excess silver halides is called fixing. The chemical used to achieve this is sodium (or ammonium) thiosulphate (sometimes called hypo). The gelatine binder which holds the silver and silver halides becomes soft and spongy in the developer. Hardening the film serves to get rid of some of this sponginess and gives the film better resistance to the formation of water marks during drying. Hardening of the film emulsion, although desirable, is not absolutely necessary if the films are subsequently washed at a temperature of less than 25°C and dried manually If automatic dryers are to be used, however, the film emulsion will be badly damaged by the rollers if the film has not been properly hardened. For manual processing the fixer-hardener bath should be maintained at the same temperature as the developer, although in this case the temperature is not so critical. Films should be fixed for twice the clearing time (the time taken for the fixer to strip out the remaining silver halide). The clearing time for a fixer bath maintained in good condition will generally be less than 2.5 minutes at 20°C. While fixing times of 1 hour or more generally have no ill effects, overfixing should be avoided because in some circumstances it can lead to frilling, (frilling is a film artefact whereby the film emulsion becomes detached from the base). Frilling can be caused by allowing film to remain in the fixer for an extended period at high temperature, particularly where hardener has not been added to the fixer.


7-7


Washing After fixing the film must be thoroughly washed so as to remove all traces of the fixer chemicals from the emulsion. Insufficient washing will result in the formation of brownish yellow stains while over-washing can cause water marks or even frilling (see above). Adequate wash times in a running water wash vary from 10 minutes at 30°C to 30 minutes at 10°C. Most film manufacturers recommend that the wash temperature should not be more than 25°C. Film can be washed successfully in a still water bath provided that the water is changed regularly. Drying The application of a wetting agent to the film prior to drying will help the film to dry quickly/evenly without watermarks. If the films are to be dried using a warm air draught then care must be taken to ensure that dust is not blown onto the wet films. Warm air dryers with a downward draught dry the film more quickly. 7.4

Advanced imaging techniques

7.4.1

Computed radiography Computed radiography (CR) uses very similar equipment to conventional radiography except that in place of a film to create the image, an imaging plate is used. The imaging plate contains photostimulable storage phosphors, which store the radiation dose received at each point in local electron energies. When the plate is put through the scanner, a scanning laser beam causes the electrons to relax to lower energy levels, emitting light that is measured to compute the digital image. Hence, instead of taking a film into a darkroom for developing in chemical trays, the imaging plate is run through a computer scanner to read and digitise the image. The image can then be viewed and enhanced using software that has functions very similar to conventional imageprocessing software, such as contrast, brightness, and zoom. Computed radiography is commonly distinguished from digital radiography (DR aka direct). Both systems require a short burst of radiation. The difference is that on exposure a DR system will almost instantly display the image on the screen in front of the radiographer, therefore removing any need for processing. Post production can, of course, be performed on DR images in the same way as for CR images.

7.4.2

Computed tomography The technique of tomography involves passing a series of X-rays through an object, and measuring the change in intensity or attenuation by placing a series of detectors on the opposite side of the object from the X-ray source. The measurements of X-ray attenuation are called projections and are collected at a variety of angles. Computer tomography (CT) is a powerful NDT technique that uses a computer to produce 2D cross-sectional and 3D images of an object from X-radiographs. Characteristics of the internal structure of an object such as dimensions, shape, internal defects and density are readily available from CT images.


7-8


7.4.3

Real-time radiography (fluoroscopy) Real-time radiography (RTR), or real-time radioscopy, is NDT method whereby an image is produced electronically, rather than on film, so that very little lag time occurs between the item being exposed to radiation and the resulting image. In most instances, the electronic image that is viewed results from the radiation passing through the object being inspected and interacting with a screen of material that fluoresces or gives off light when the interaction occurs. The fluorescent elements of the screen form the image much as the grains of silver form the image in film radiography. The image formed is a positive image since brighter areas on the image indicate where higher levels of transmitted radiation reached the screen. This image is the opposite of the negative image produced in film radiography. In other words, with RTR, the lighter, brighter areas represent thinner sections or less dense sections of the test object. Real-time radiography is a well-established method of NDT having applications in automotive, aerospace, pressure vessel, electronic and munition industries, among others. The use of RTR is increasing due to a reduction in the cost of the equipment and resolution of issues such as protecting and storing digital images.


7-9


Section 8 Production of a Radiograph (Film Radiography)

8

Production of a Radiograph (Film Radiography) A radiograph is a record of the way in which a beam of radiation has been differentially absorbed by an object stored on photographic film. In order to produce good quality radiographs economically numerous factors have to be taken into account. First it is necessary to understand what is meant by radiographic quality.

8.1

Radiographic quality The quality or sensitivity of a radiograph is a measure of the ability of the radiograph to detect small changes in radiation intensity caused by variations in object thickness or composition. In order to detect small imperfections in an object an adequate level of sensitivity must be achieved. The standard methods of measuring or estimating sensitivity are described in the next chapter. Achieving adequate sensitivity is the crucial factor which determines the level of success of radiography as an NDT technique. Figure 8.1 summarises the factors which must be considered if radiographs of adequate quality are to be produced. Radiographic quality or sensitivity depends on achieving good contrast and good definition. Radiographic sensitivity Contrast

Definition

Figure 8.1 Factors affecting radiographic sensitivity.

8.1.1

Contrast Contrast can be defined as the ease with which it is possible to distinguish between two adjacent areas of different film density. The chief factor which determines whether or not the two areas will be clearly defined is the degree of difference in film density. Radiographic contrast is formed by: 1 2

The object being radiographed: Subject contrast. The film used to produce the radiograph: Film contrast.

NDT20-71015 Production of a Radiograph (Film Radiography)

8-1


Subject contrast can be defined as the degree of difference in transmitted radiation intensity produced by a given change in subject thickness. This is primarily a function of the type of material from which the subject is made. For instance a 1mm step in a 10mm section of lead will produce a much greater change in transmitted radiation intensity than would the same step in a similar section of aluminium (assuming that the energy of the incident radiation was the same in both cases). Film contrast can be defined as the degree of difference in film density produced by a given change in radiation intensity or exposure time. This is primarily controlled by the type of film used. The factors affecting film and subject contrast are discussed below. Film type (affects film contrast) In considering the effect that film type has upon film contrast (the type of film has no effect on subject contrast) it is useful to refer to a type of graph called a film characteristic curve. Such graphs relate the logarithm of the relative exposure time to the achieved film density; an example is given in Figure 8.2. The gradient of a film characteristic curve represents the change in film density produced by a small change in subject thickness. Figure 8.3 shows how the gradient of the film characteristic curve varies with film density. Note: The curve for Kodak MX125, an ultrafine grain film, has the steepest gradient. The Agfa D7 curve is in turn steeper than that of Kodak CX. D7 and CX are both class C5 fine grain film, but CX is slightly faster film than D7. Thus MX125 will provide the best film contrast, whilst D7 should produce contrast better than that of CX. Film density (affects film contrast) Film density can be defined as the degree of darkening of the film or more properly the degree to which the film prevents light from passing through it. Mathematically, film density is defined as the logarithm to the base 10 of the ratio of the incident to transmitted light intensity. It can be calculated using the following formula:

The logarithm to the base 10 of a number is just the power of 10 that will produce the number itself. For example: 102 = 100 and the logarithm of 100 = 2. 103 = 1000 and the logarithm of 1000 =3.


8-2


Film density

Log10 relative exposure

MX

6.0

Gradient of film characteristic curve increasing gradient = increasing film contrast

8.0

Figure 8.2 Film characteristic curves Kodak CX, AGFA D7 and Kodak MX125 (direct type film/lead screens).

0.0

2.0

4.0

D7 CX

0.0

1.0

2.0

3.0

4.0

Film density Figure 8.3 Gradient of the film characteristic curve versus film density for Kodak MX125, Agfa D7 and Kodak CX.


8-3


Thus a film having a density of 2.0 transmits 1% of the incident light intensity while a film with a density of 3.0 transmits only 0.1%. A film with a density of 0.3 would transmit about 50% of the incident light intensity. Figures 8.2 and 8.3 show how film density affects film contrast. Film density does not affect subject contrast. The gradient of the film characteristic curve is a good measure of film contrast. The gradient for all films increases with increasing film density. If the gradient is steep then a small change in radiation intensity or exposure time will produce a large change in film density. The gradient of all of the film characteristic curves becomes shallow at film densities of less than 1.5, indicating that film contrast will be poor at low film densities. In view of this all relevant industrial standards stipulates a minimum film density for industrial radiography. Base fog level (affects film contrast)

Film density

Standards generally limit the base fog level of unexposed radiographic film to 0.3. If the base fog level exceeds this value film contrast can be quite severely affected. Fog level can be checked by processing a sample of the unexposed film. Figure 8.4 demonstrates how the base fog level affects film contrast.

Log10 relative exposure Figure 8.4 Effect of film fogging on the film characteristic curve.

The dotted lines show the average gradient between film densities 1.5 and 2.5 for film having a base fog level of 0.1 and 0.5 respectively. The average gradient with a base fog level of 0.1 is about 3.6 while that for a base fog level of 0.5 is about 2.7. This decrease in average gradient is indicative of a reduction in film contrast.


8-4


Film processing (affects film contrast) Radiographic film should always be processed in accordance with the manufacturer’s recommendations. Any deviation from these will result in a lowering of film contrast and hence sensitivity. The film attains about 80% of its final density in the first 30 seconds of development. During the remaining 3½-5½ minutes of the standard development time radiographic developers are designed to increase film contrast. The developer works more vigorously in areas where a lot of metallic silver has already been released. Thus film contrast gradually improves during the final minutes of the development process. This is why radiographs which have been pulled intentionally underdeveloped in an effort to produce acceptable film density invariably show poor film contrast. If the film is allowed to remain in the developer for too long, however, the developer will begin to attack all areas of the film and contrast will begin to suffer. All radiographic developers are designed to for use at the processing temperature specified by the developer manufacturer. Development time can, to some extent, be increased to compensate for a lower developer temperature or reduced if the temperature is above the optimum, but this will invariably be at the cost of reduced film contrast. In order to maintain the developer in good condition it must be replenished. Developer which has not been properly replenished quickly leads to low contrast low quality radiographs. Radiation quality (affects subject contrast) As the photon energy of the incident beam of radiation increases the subject contrast produced by the same change in component thickness decreases. This is because higher energy radiation is less absorbed as it passes through a given thickness of the same material than is lower energy radiation. It is useful to talk about different radiation energy in terms of its half value layer. The half value layer can be defined as the thickness of any particular material which will reduce the intensity of the incident radiation by a factor of two. The thickness of the half value layer for any material increases with increasing radiation energy. Examples of half value layers for various materials and radiation energies are given in Table 8.1. Table 8.1 Half value layers. Nature of incident radiation Iridium 192 -rays Cobalt 60 -rays 100kV X-rays 150kV X-rays 200kV X-rays 300kV X-rays

Half value layer, mm Steel Aluminium, 13 35 22 70 2.4 15 4.5 18.5 6 21 8 25

Lead 4.8 12.5 0.1 0.3 0.65 1.6

Note: That as half value layer decreases subject contrast increases.


8-5


Scatter (affects film and subject contrast) As ionising radiation passes through any material it undergoes a process known as scattering. Scattering occurs due to various mechanisms (the principal cause varies with the radiation energy) all of which occur due to the way in which radiation photons interact with atoms. When an X-ray photon strikes an atom it will cause the atom to lose one or more electrons, so that the affected atom becomes positively charged. Such electrically charged atoms are normally referred to as ions. Ions, by their nature, are not stable, they will try to attract electrons into their empty energy shells in order to achieve a zero electrical charge. As electrons are captured from free space by ions they give up part of their kinetic energy as a photon of radiation. These photons will radiate in all directions from the affected atoms. Such radiation is known as scattered radiation and can lead to an overall fogging of the film emulsion. This reduces film contrast. Scatter can severely reduce subject contrast by reducing the differences in radiation intensity reaching the film from various parts of the component under test. Scattering mechanisms and methods of controlling scattered radiation are discussed in a later section. 8.1.2

Definition Definition is a measure of the sharpness of the images on the radiograph. It can be defined as the width of the boundary between two areas of different density on a film. The opposite of definition is unsharpness. An illustration of unsharpness is given in Figure 8.5. The total unsharpness on a radiograph is due to three factors: Geometric unsharpness (also called penumbra or penumbral shadow). Inherent unsharpness (also called film unsharpness). Relative movement during exposure. These are described and discussed below.

Increasing unsharpness

Figure 8.5 Unsharpness: The unsharpness of the boundary between light and dark increases from left to right.


8-6


Geometric unsharpness A major factor affecting definition on a radiograph is geometric unsharpness. This can be defined as the width of the penumbra. Penumbra is the word used in physics to describe the lack of sharpness at the edge of a shadow. Umbra means full shadow and penumbra means half shadow. So long as the source of radiation is not a true point source (and it never is a true point source) there will be a penumbral area at the edge of any shadow. A radiographic image is basically just a shadow. The focal spot of an X-ray tube and a radioactive isotope always have finite physical dimensions so a penumbra is always produced. Once the achieved penumbra falls below about 0.2mm the unaided human eye ceases to perceive any further improvement in definition. For very high quality radiographic techniques geometric unsharpness is therefore generally kept to a value of less than 0.2mm. To achieve this, the object to film distance is kept short and the radiation source to film distance is made as long as necessary depending upon the radiation source dimensions.

Ug 

f OFD  FOD 

Figure 8.6 Geometric unsharpness.

Inherent unsharpness Inherent unsharpness depends on three factors: the type film, the type of intensifying screen and the quality (or photon energy) of the radiation. Film (Effect on inherent unsharpness) The emulsion of any radiographic film is made up of silver halide grains. As already mentioned the size of these grains is varied in order to control the film speed. Each grain of silver halide needs to interact with as few as a single photon of ionising radiation in order to become sensitised. Sensitised grains of silver halide are very rapidly reduced to silver metal on contact with the reducing agents contained in film developer. Obviously if the grains of silver halide are large they will tend to cause a blurring effect on the radiographic image - a similar effect occurs on a computer screen. At a resolution of 640 x 480 the image quality is quite poor; this is like a coarse grain film. At a resolution of 1024 x 768 the image quality is considerably better because the pixel size is much smaller. This is like a fine grain film.


8-7


Each silver halide grain in a fine grain film is around 1μm in size. To give an idea of just how small this is, the pixel size on a computer screen at a resolution of 1024 x 768 is in excess of 40μm. Quality of radiation (Effect on inherent unsharpness) When ionising radiation passes through a material (including radiographic film) it is scattered. Scattering processes involve the emission of electrons, so called secondary electrons. As the radiation energy of the primary beam increases the kinetic energy of the secondary electrons released also increases. As the velocity of the secondary electrons increases they become capable of penetrating an ever-increasing thickness of the surrounding material. The reason why lead screens produce no intensification effect below 120keV is that the below this primary beam energy the secondary electrons released in the screens have insufficient energy to penetrate the supercoat of the film. It is now known that most silver bromide grains in a direct type radiographic film are not sensitised directly by the penetrating radiation itself. They are for the most part sensitised by the secondary electrons released by the intensifying screens and by secondary electrons generated within the film emulsion itself. The greater the distance the secondary electrons are able to travel within the emulsion the greater the resulting unsharpness. At very high radiation energies (exceeding 1.02MeV) a scattering process called pair production begins to predominate (pair production is more fully described in Section 8.2.1). Pair production releases high energy electron positron pairs. The positron released is annihilated as it meets with a free electron. This produces a burst of X-rays with a characteristic photon energy of 0.51MeV. This so-called annihilation radiation greatly reduces the sharpness of the image. The bremstrahlung released as the electron half of the pair collides with neighbouring shell electrons further reduces image sharpness. Intensifying screens (Effect on inherent unsharpness) Lead or copper intensifying screens increase the film speed by producing secondary electrons which are capable of sensitising adjacent silver halide grains. As the radiation energy increases the energy of the electrons produced by the screens increases and this leads to an increase in inherent unsharpness because the electrons are capable of travelling longer distances within the film emulsion. Unsharpness will be increased still further if the screens are not in good contact with the film. Fluorometallic screens produce light photons in addition to secondary electrons. The production of light photons inevitably produces an increase in unsharpness compared to lead or copper screens because there is no limit to how far the photons of light can travel. This loss of image quality will be greatly exacerbated if good film screen contact is not maintained. Salt screens fluoresce strongly under the influence of X-rays and produce very large increases in film speed. They are not generally used in industrial radiography due to the large increase in inherent unsharpness associated with their use. Relative movement during exposure An increase in unsharpness will also be produced if there is any relative movement between the source, object or film during exposure. This can be a particular problem when carrying out radiography of pipework which is in service (and therefore vibrating) or when radiography has to be performed in windy conditions.


8-8


8.2

Radiation scattering and scatter control Matter which has absorbed ionising radiation and which has therefore reached an unstable energy state will emit energy in the form of radiation as it returns to a stable energy state. Some of this radiation will be in the form of heat, in a few specialised cases it will be in the form of light and in many cases X-rays will be produced. Such X-rays are termed scattered radiation and they can very adversely affect radiographic quality. Control of scattered radiation is therefore essential if high quality radiographs are to be produced.

Source

Primary radiation

Scatter

Figure 8.7 Scattered radiation.

8.2.1

Scattering mechanisms – the causes of scatter There are three scattering mechanisms of particular importance in radiography - the photoelectric effect, Compton or incoherent scattering and pair production. The predominant scattering mechanism depends on the photon energy of the primary beam. Several other scattering mechanisms are possible, including Rayleigh or coherent scattering, but these are of little importance in industrial radiography. The photoelectric effect The predominant scattering mechanism below 0.6MeV is the photoelectric effect. In this all of the energy of the incident X- or gamma ray photon is transferred to an orbital electron. The absorbing electron is in most cases ejected from the atom and ionisation occurs. In a few cases where the energy of the incident photon is correct the absorbing electron merely jumps from an inner to an outer energy shell. As this electron at some later stage falls back into its original energy state a photon of X-rays is emitted. The energy of this photon is a characteristic of the scattering atom. Characteristic radiation emission can be used to perform chemical analysis. Atoms which have been ionised emit X-rays as they capture an electron from free space and the ejected electrons emit X-rays as they collide and interact with the atoms in their path.


8-9


E0

E 0 - Eb

Figure 8.8 Photoelectric effect. An incident X-ray photon, energy E0 collides with an outer shell electron which is ejected from the atom with energy E0 – Eb where Eb is the binding energy.

Compton scattering (incoherent scattering) The predominant scattering mechanism above 0.6MeV and up to 6MeV is Compton scattering. In this only part of the energy of the incident X- or gamma ray photon is transferred to an orbital electron. The absorbing electron is ejected from the atom and ionisation occurs. The remaining photon energy continues as a lower energy X-ray although slightly deflected from its original path. Atoms which have been ionised emit X-rays as they capture an electron from free space and the ejected electrons emit x-rays as they collide with and interact with the atoms in their path.

Ee

E0

E e E0

Figure 8.9 Compton scattering.

Pair production The predominant scattering mechanism above 6MeV is pair production. In this the incident X- or gamma ray photon collides either with the nucleus or an inner shell electron. The incident photon then converts to an electron - positron pair. A positron is a particle having the same size and mass as an electron but opposite electrical charge. Pair production cannot occur below a threshold photon energy of 1.02MeV. The electron-positron pair is ejected at high velocity but the positron has a very short life. It quickly meets a free electron and annihilation occurs - the positron and electron cease to exist and 2 photons of 0.51MeV radiation are emitted. The ejected electron emits X-rays as it collides and interacts with the atoms in its path.


8-10


E0 2X0.51MeV

E+

EFigure 8.10 Pair production.

Total scatter at different primary beam energies

Photoelectric

Compton Pair production Total

Figure 8.11 Total scatter versus radiation energy.

Figure 8.11 shows how scatter is a greater problem at low incident radiation energy. Scatter as a percentage of the total radiation is at a minimum at around 2MeV, however, as radiation energy increases through the threshold photon energy of 1.02MeV pair production within the film emulsion begins to increase inherent unsharpness resulting in poor image quality. Usually a decrease in incident radiation energy would be expected to produce improved subject contrast and reduced inherent unsharpness. There is a limit to this, however, where the increased scatter associated with low incident radiation energy begins to counteract the beneficial effect of reducing the incident radiation energy and a decrease in image contrast may be the overall result.


8-11


8.2.2

Types of scatter Several types of scatter cause problems in radiography ─ side, back and internal scatter (self-scatter). The angle formed between the direction of travel of the primary beam and the scattered radiation (reaching the film) is called the scattering angle or angle of scatter. Side and internal scatter have an angle which is less than or equal to 90 while for back scatter the angle exceeds 90. Side scatter Radiation may be scattered by parts of the object that are not within the diagnostic area of the radiograph or by the walls of the exposure room. This is termed side scatter. This type of scatter can be reduced by collimating the beam such that only the area to be examined is subjected to the primary beam and by the use of lead masking, diaphragms or grids. In X-radiography the use of a filter may help to reduce side scatter. Side scatter causes undercutting of the radiographic image around the edges of a component where these can be seen on the radiograph or at any site where there is a large change in section thickness (eg a bolt hole). Undercutting causes a lack of sharpness and may mask possible defect indications. Back scatter Back scatter is caused by the primary beam striking an object behind the film and scattering back. It can easily be reduced by shielding the back of the film cassette with a sheet of lead, approximately 2mm thick is adequate for most applications. In Xradiography the use of a filter may help to reduce back scatter. The presence of excessive back scatter may be detected by placing a lead letter B on the back surface of the cassette (ie the cassette surface furthest from the source of radiation). If there is excessive back scatter then a light image of the letter B will be seen on the developed film. The use of a lead letter B is mandatory when working in accordance with the ASME code and is required for each new technique by EN ISO 17636-1 (ie not for production radiography). In accordance with EN ISO 17636-1 the lead letter B shall be a minimum of 10mm high and 1.5mm thickness. Note: If a dark image of the letter B appears this is not excessive back scatter. It merely indicates characteristic radiation caused by the letter B itself. Should back scatter be detected then the thickness of the lead sheet shielding the back of the film cassette must be increased. Self-scatter Self-scatter is scattered radiation originating from within the test component. The detrimental effect on film quality can be reduced by the use of lead intensifying screens placed in contact with the film and, in X-radiography, by the use of filters.


8-12


If the radiation source is an X-ray tube then the use of a copper filter can help to reduce the effects of this type of scatter. A copper filter significantly reduces the proportion of low energy radiation within the primary beam. Since it is the low energy radiation which is chiefly responsible for scatter the use of such a filter can reduce the overall amount of scatter occurring and in this way improve image quality. Filters made from lead, steel or other metals may be used in a similar way. Metallic foil intensifying screens made from lead or other metals reduce the effects of self-scatter for both X- and gamma ray radiography as they filter out the low energy scattered radiation and prevent it from reaching the film. 8.2.3

Scatter control Collimation Probably the single most effective way of reducing scatter is to collimate the radiation beam. Collimators shield out most of the radiation which is not travelling in the useful direction. X-ray equipment is always to some extent selfcollimated – the geometry of the hooded anode shields out much of the unwanted radiation produced, but some X-ray heads may contain additional shielding. In gamma radiography collimators consisting of hollowed out blocks of lead or tungsten. The principle of collimation is simply that if there is less radiation then there will be proportionally less scatter. Diaphragms Diaphragms take collimation a step further. They consist of a sheet of lead which has a hole cut in it the same shape as the object which is being radiographed. Using a diaphragm the radiographer is attempting to shield out all unwanted radiation, the set up for radiography must however, be extremely accurate if it is to be successful. Diaphragms are therefore more likely to be seen where a fully automated technique is in use that allows for a very high degree of repeatability in the set-up accuracy. Masking or blocking Masking or blocking consists of placing sheets of lead, bags of lead shot or barium putty or any other radiation absorbing material around the object which is being radiographed to reduce the undercutting effect of side scatter. Figure 8.13 below shows the benefits of blocking. Focus or source Diaphragm

Object Film

Figure 8.12 Using a diaphragm.


8-13


Undercutting

3mm lead blocking

Figure 8.13 Radiographs produced with and without blocking.

Grids The use of a grid is generally limited to medical radiography. It consists of a matrix of parallel metal bars set in oscillation during exposure such that the grid itself does not produce a radiographic image. It grid is a very effective method of reducing the effects of side scatter, but grids are very rarely a practical option for industrial situations. In order to be effective the grid must be placed as close as possible to the film and microfocus X-radiography it may be placed between the film and the object. Filters Figure 8.11 shows how the percentage of scattered radiation is high when the radiation energy is low. Placing a thin sheet (typically 1 to 2mm) of copper or other metal in the primary beam, close to the source of radiation, greatly reduces the amount of low energy radiation while permitting most of the higher energy radiation to pass through. If there is less low energy radiation there will be less scatter, although it is possible that film contrast will be reduced. The use of a filter to reduce scatter is limited to X-radiography because gamma ray sources do not produce long wavelength low energy radiation.

Closely spaced steel slats absorb radiation other than the primary beam

Oscillating Figure 8.14 Oscillating grid.


8-14


Figure 8.15 The effect of filtration on a typical X-ray spectrum.

Metallic foil screens Lead screens, or those made from other metals such as steel or copper are a very effective means of reducing scatter, for both X- and gamma radiography, particularly when the energy of the primary beam exceeds 120keV. Such screens are placed in contact with the film and the front screen works like a filter, greatly reducing the proportion of low energy radiation which reaches the film. Scattered radiation is always lower energy than the primary beam, so the scatter is more affected by the filtration effect than is the primary beam. The back screen reduces back scattered radiation which reaches the film. In addition to this both screens intensify the effect of radiation, the energy of which exceeds 120keV. The screens do this by producing secondary electrons to which the film emulsion is sensitive. Most of the radiation exceeding 120keV will be part of the primary beam. Thus the effect of the primary beam is amplified at the expense of the unwanted scattered radiation.

Pb screens, good contact

No screens

Figure 8.16 The effect of lead screens.

Higher radiation energy Up to a radiation energy of around 1.5MeV increasing the maximum radiation energy of the primary beam will reduce scatter (in proportion to the primary beam) and improve image quality. At higher radiation energy there is a continued decrease in the proportion of scattered radiation, but pair production within the film emulsion begins to have an increasingly detrimental effect on film unsharpness, Figure 8.11 above. Change from X- to gamma-ray radiography The absence of low energy components in radiation obtained from radioactive materials such as iridium 192 or cobalt 60 is the reason why gamma ray radiography is much less affected by scatter than X-ray radiography.


8-15


Reducing focus or source to film distance The use of a source to film distance that will produce a value of geometric unsharpness which is less than the inherent unsharpness for the film-screen combination in use is therefore not recommended. 8.3

Determining the correct exposure: Exposure charts Exposure in film radiography can be defined as the amount of radiation striking or passing through the film. The amount of radiation striking the film is equal to the product of the radiation intensity* at the film and the exposure time. Exposure is critical, because underexposed films will show low film density and therefore reduced contrast, while overexposed films which exceed a film density of about 4.0 cannot usually be satisfactorily viewed using the unaided human eye and standard film illuminators. * Note: In film radiography it is better to think of intensity as photons per square metre rather than energy per square metre. This is because radiographic film has a fairly flat response to changes in photon energy right across the full spectrum of X- and gamma rays. In terms of film density 1 photon of 150keV radiation has much the same effect as 1 photon of 20MeV radiation. In X-radiography the number of photons produced per unit time is directly proportional to the tube current. Therefore it is usual to express X-ray exposures in milliampere-minutes (mA-min); the product of the exposure time and the tube current. In gamma radiography the number of photons produced per unit time is directly proportional to the source activity. Source activity is usually measured in curies (Ci) or, less commonly in gigabecquerels (GBq). Therefore gamma ray exposures are usually expressed in curie-minutes (Ci-min) or curie-hours (Ci-h) but may also be expressed in gigabecquerel-minutes (GBq-min) or gigabecquerel-hours (GBq-h). The factors listed in Table 8.2 will affect either the film speed or the amount of radiation reaching the film and have to be taken into account when determining the correct exposure for film radiography. In addition to these factors the required film density obviously has an impact upon the required exposure time.

8.3.1

Exposure charts Exposure charts provide a convenient means of estimating radiographic exposures for both X-ray and gamma ray techniques. All exposure charts are correct only for a fixed set of conditions - all of the factors mentioned in Table 8.2 are fixed for any particular chart. Exposure charts for X-ray equipment are usually applicable only to a single type of equipment. Figures 8.17-8.19 show an example of X-ray exposure charts. Figure 8.20 is an example of an exposure chart for iridium 192.


8-16


Table 8.2 Factors affecting radiographic exposure (film radiography) Factors affecting film speed (Table 8.2 Part 1) Factor Film type

Intensifying screen type

Film processing

Radiation energy

Comments Coarse grained films are fast and require a short exposure while fine grained films are slow and require a long exposure. The effect of intensifying screens varies with the incident radiation energy. Metallic foil intensifying screens reduce the required exposure time by a factor of about 2 or 3 at radiation energies of 120keV or more. Fluorometallic screens reduce the exposure time by a factor of about 50 with films designed for use with such screens. Salt screens can reduce the required exposure time by a factor of 500 but are seldom used for industrial radiography. Developer type and concentration together with the development temperature can affect the film speed. Automatic processing usually gives a slight increase in film speed when compared with manual processing. Fairly minor compared with other factors. Can affect the efficiency of the intensifying screens, and to a lesser extent the film speed.

Factors affecting the intensity of radiation reaching the film (Table 8.2 Part 2) Comments The amount of radiation absorbed by a material increases with Material type increasing density and atomic number. The amount of radiation absorbed by an object rises exponentially with increasing material thickness. Factor

Material thickness

Radiation energy

Radiation energy distribution

Source or focus to film distance

Filters Source strength or tube current Factor Film density

∙



Where I0 = the intensity of the incident radiation; I = the intensity of the transmitted radiation;  = the coefficient of linear absorption; t = the penetrated thickness The amount of radiation absorbed by a material decreases with increasing radiation energy. For X-ray techniques the amount of radiation which will be absorbed for a given radiation energy cannot be calculated - this has to be measured experimentally. Exposure times for self-rectified X-ray equipment are always longer than those for constant potential equipment operating at the same peak tube voltage. The radiation produced by constant potential equipment is said to be ‘harder’ because it contains proportionally more high energy radiation. In fact radiation energy distribution can vary quite markedly even between different types self-rectified or constant potential unit. Xray exposure charts therefore are applicable to only a single equipment type. The amount of radiation reaching the film is proportional to the reciprocal of the square of the source to film distance. As the required exposure time is inversely proportional to the radiation intensity: E al D2 Where E is the exposure time at distance D. If the half value layer of the material from which the filter is made is known and its thickness is known then it is possible to compensate the exposure for the insertion of or removal of a filter. Radiation intensity is proportional to the tube current for x-rays and to the source activity for gamma rays. Film density (Table 8.2 Part 3) Comments The required exposure is strongly dependent upon the required film density. Film characteristic curves can be used to compensate for a change in the required film density.


8-17


Steel thickness/mm

Exposure/mAmins

Exposure/mAmins

Andrex 140kV Kodak CX FFD 914mm Lead screens 0.125mm Density 2.0

Steel thickness/mm Figure 8.17 Exposure chart for Andrex 140kV SR directional.


8-18


Steel thickness/mm

Exposure/mAmins

Exposure/mAmins

Philips 300kV X4 Kodak CX FFD 1000mm Pb screens 0.125mm Standard development Density

Steel thickness/mm Figure 8.18 Exposure chart for Philips 300kV SR directional.


8-19


Exposure/mAmins

Exposure/mAmins

Steel thickness/mm

Pantak 200kV Kodax CX FFD 914mm Lead screens Density 2.2

Steel thickness/mm Figure 8.19 Exposure chart for Pantak 200kV CP directional.


8-20


Steel thickness, mm

Exposure/curie hours

Exposure/curie hours

Source - Iridium 192 Development - 4min at 20oC Source to film distance - 900mm Material - steel Screens - lead Film - Kodak CX

Steel thickness.mm

Figure 8.20 Exposure chart for Iridium 192.


8-21


8.3.2

Using exposure charts (X-ray) Focus to film distance Two factors affect the choice of focus to film distance, geometric unsharpness (Ug) requirements and the desired diagnostic film length (DFL). EN ISO 17636-1 does not directly specify geometric unsharpness, but controls it by specifying a minimum value of focus to film distance for a given effective focus size and object to film distance, see Figures 8.22 and 8.23 below. Example:

Figure 8.21 Butt weld in 15mm plate. Radiograph of a butt weld in 15mm thickness steel plate (allows for a weld reinforcement of 3mm).

Taking the example given in Figure 8.21 above let’s suppose that we wish to achieve a geometric unsharpness of 0.25mm or better. This would be sufficient to satisfy the requirements of most national codes or standards including EN ISO 17636-1 class A.

Ug 

f OFD  FOD 

but the OFD can be taken as being equal to the material thickness, in this case 18mm, and the FFD is equal to the FOD plus the material thickness so:

0.25 

f  18 FFD  18

If we choose to use the Pantak 200 CP the effective focus size on the broad focus setting is about 4mm, so:

Minimum FFD 

4  18  18  306mm 0.25


8-22


Source size: 3mm

Minimum source to film distance, mm

Minimum source to film distance, mm

Object to film distance, mm

Note: This diagram is for an effective source size of 3mm. The minimum SFD for other source sizes can be found as follows: Find the minimum SFD for the desired image class from the figure, let this distance = d Minimum SFD for source size = f is then equal to: f (d-OFD)+OFD

Object to film distance, mm Figure 8.22 Minimum SFD/FFD.

Geometric unsharpness at minimum source to film distance, mm

Note: EN ISO 17636-1 standard (basic) techniques are Class A, while enhanced (improved) techniques are Class B.

Object to film distance, mm Figure 8.23 Geometric unsharpness at minimum SFD/FFD.


8-23


So a focus to film distance of 306mm will achieve the required value of geometric unsharpness. However if we use this minimum FFD the diagnostic film length (DFL) will be rather short due to fade off. EN ISO 17636-1 has a requirement for Class B techniques that the penetrated thickness (based on the nominal thickness) at the ends of the DFL shall not exceed 110% of the nominal thickness. For Class A the requirement is that the penetrated thickness shall not exceed 120% of the nominal thickness at the end of the DFL. This translates to a DFL that is approximately 0.9 x FFD for Class B and 1.3 x FFD for Class A. In this case let’s apply the EN ISO 17636-1 Class A requirement. If we want to achieve a DFL of 450mm the minimum FFD based on this will be 450/1.3 = 346. For convenience we can round this up to say 400mm. Tube voltage After deciding on an appropriate value for focus to film distance the next thing to consider when determining an exposure time for an X-ray technique is what tube voltage will be appropriate. Some standards specify the maximum tube voltage which may be used for a given thickness of material. EN ISO 17636-1 requirements, the maximum tube voltage in our case would be about 245 kV. However, common good practice, which is to choose a tube voltage which will produce an exposure time of between 1 and 5 minutes at around 75-100% of the maximum tube current, will, in nearly all cases, satisfy such codes and standards. The exposure chart for the Pantak 200 CP is drawn for a source to film distance of 914mm. The maximum tube current is 14mA. Let’s use 10mA. We would like to achieve an exposure time of between 1 and 5 minutes, giving an exposure of between 10 and 50mA.mins. The focus to film distance that we wish to use is 400mm. The exposure chart has been constructed using FFD = 914. Using the inverse square law (see Figure 8.24) we can see that an exposure of 10 to 50mA-mins at 400mm FFD is equivalent to an exposure at 914mm FFD of between 10 x 9142/4002 = 52.2mA-mins and 50 x 9142/4002 = 261mA-mins. Looking at the exposure chart (see Figure 8.19) a density of 2.2 will be achieved using Kodak CX with an exposure of about 150mA-mins at 120kV or with an exposure of about 40mA-mins at 140kV. These values are for an FFD of 914mm. The equivalent exposures at an FFD of 400mm will be: 150 x 4002/9142 = 29mA-mins at 120kV or 40 x 4002/9142 = 7.66mA-mins at 140kV. Thus for Kodak CX and a film density of 2.2 these exposures should work: 120kV, 10mA, 2 minutes 54 seconds or 140kV, 6mA, 1 minute 16 seconds. The 120kV exposure should produce the best film contrast while the 140kV exposure will be more economic.


8-24


Figure 8.24 The inverse square law.

Intensity = number of photons per square metre. At 2D the same number of photons that passed through one square at D now passes through 4 squares. Thus the intensity at 2D is one quarter of what it was at D. Intensity is proportional to 1/(distance)2. Radiographic exposure time is proportional to 1/(intensity), thus exposure time is proportional to (distance)2. The inverse square law can be stated as follows: 2

 Old     Old radiation int ensity   dis tan ce  New radiation intensity or dose rate =   2  or dose rate   New     dis tan ce  Or 2

 New    dis tan ce   New exposure = (Old exposure) 2  Old     dis tan ce 


8-25


Changing the film density If a film density of 2.2 was thought too light, then the film characteristic curve (Figure 8.2) can be used to find the correct amount of exposure compensation. Let’s say that we wanted to achieve a film density of 2.5. Using Figure 8.2 logarithm (relative exposure) for a film density of 2.2 on CX = 1.3 while for a film density of 2.5 logarithm (relative exposure) = 1.38 1.38 – 1.3 = 0.08 Antilogarithm (0.08) = 100.08 = 1.2 So at 120kV the exposure required = 1.2 x 29 = 35mA-mins while at 140kV it would be 1.2 x 7.66 = 9.2mA-mins Thus if we use Kodak CX film the following exposures should achieve a film density of 2.5: 120kV, 10mA, 3 minutes 30 seconds or 140kV, 6mA, 1 minute 32 seconds Changing the film type Sometimes we may wish to change to a different type of film. For example if the required radiographic sensitivity could not be achieved using Kodak CX film we might consider using Kodak MX125 film which should produce better contrast and consequently better sensitivity. This can be achieved using the film characteristic curves, but it is more convenient to use film factors. Table 8.3 lists film factors for some common direct type X-ray films. Table 8.3 Film factors Classification


EN ISO 11699-1

ASTM E 1815

C1 C2 C3 C4 C5 C6

Special Class 1 Class 2 Class 3

AGFA

CF

KODAK

CF

FUJI

CF

FOMA

CF

D2 D3 D4 D5 D7 D8

9.0 4.2 2.6 1.6 1.0 0.7

DR 50 M 100 MX 125 T 200 AA 400 CX

7.2 4.2 2.8 1.7 1 0.7

IX 25 IX 50 --IX 80 IX 100 IX 150

6.5 3.3

R2 R3 R4 R5 R7 R8

9.0 4.2 2.6 1.6 1.0 0.7

1.6 1.0 0.6

Note: The film factors given in Table 8.3 are approximately correct for radiation energy in the range 0.1 to 1.0MeV. Film factors can vary with radiation energy.


8-26


Suppose that we wish to change from film A to film B:

Old exp osure  Film factor A

Thus to change from CX to MX125 we divide exposure time by the film factor for CX (CF = 0.7) and multiply the result by the film factor for MX125 (CF = 2.8). Original radiograph (with density 2.5) was exposed for time 1 minute. If we wish to produce radiograph with same density on film Kodak MX125, than the exposure time shall be ca. 4 minutes (considered that other parameters are constant. Radiography of other materials All of the exposure charts in the figures above have been constructed for steel. If radiography has to be carried out on materials other than steel then the exposure time will have to be adjusted to compensate for the difference in radiation absorption. This can be done using half value layers. The half value layer of a given material for a given incident radiation energy is the thickness of the material which reduces the intensity of the incident radiation by a factor of 2. However the simplest way is to use equivalence factors, some examples of which are listed in Table 8.4. Table 8.4 Equivalence factors.

Material Steel Copper Aluminium Al alloy 4.5% Cu Titanium

Radiation energy/isotope 100keV 150keV 220keV 1.0 1.0 1.0 1.5 1.6 1.4 0.08 0.12 0.18 0.13 0.16 0.22 0.5 0.45 0.35

400keV 1.0 1.4 -

Ir192 1.0 1.1 0.35 0.35 -

Going back to the example, if we needed to radiograph a weld dimensionally similar but made in a copper-based alloy rather than steel then the first thing would be to work out the steel equivalent thickness. This is equal to the actual thickness of copper divided by the copper equivalence factor and multiplied by the equivalence factor for steel. Stated generally this is: From the table it can be seen that at 150kV the copper equivalence factor is probably about 1.6, while that for steel is 1.0, so 18mm copper is radiographically equivalent to 18 ÷ 1.0 x 1.6 = 28.8mm steel. Looking at Figure 8.19 it can be seen that an exposure of about 205mA-mins would be required using the Pantak 200 CP to achieve a film density of 2.2 at a FFD of 914mm using CX film on 18mm thickness copper (28mm steel equivalent).


8-27


Compensating for the use of a filter If problems arise with scattered radiation one possibility in X-radiography is to use a filter. Typically filters made from 1 or 2mm thickness copper sheet are used, filters made from other metals such as lead may also be used. The difference which the use of a filter will make to the exposure time for the steel weld in the example can be calculated using the equivalence factors in Table 8.4. Suppose that we wish to radiograph our 18mm thickness steel weld using the Pantak 200 at 140kV with a focus to film distance of 400mm. The exposure required for a film density of 2.5 without a filter was calculated above as 9.2mA-mins if using CX film. The equivalence factor for copper at 140kV is about 0.64. Therefore 1mm of copper will be radiographically equivalent to 1 ÷ 0.64 x 1 = 1.6mm of steel. To find the correct exposure (for a copper filter thickness of 1mm) we simply need to add this amount to the steel thickness which is being radiographed: 18 + 1.6 = 19.6mm Figure 8.19 gives an exposure of about 50mA-mins for 19.6mm of steel at a focus to film distance of 914mm, which is an increase of 25% compared to the exposure which was required under the same conditions without the filter. Thus the exposure required with a FFD of 400mm using CX film for a film density of 2.5 increases by 25% from 9.2 to 11.5mA-mins. Other possible changes The exposure charts (Figures 8.17 to 8.19) were made using lead screens, 0.125mm thickness and automatic film processing. If either or both of these two factors is changed then the only way to establish the correct exposure will be by experimentation, although the charts will still act as a guide. The exposure charts in Figures 48 to 50 are fixed for a particular type of X-ray equipment. They cannot be used to predict exposures for any other type of Xray equipment. Gamma-ray exposures The method used to establish gamma-ray exposures from an exposure chart is similar to that used for X-rays except that the possibility to change the radiation energy has been removed. Before carrying out gamma radiography it will be necessary to establish the source activity at the time of exposure. This is done by reference to the decay chart supplied by the source manufacturer. Other convenient means of establishing gamma ray exposures are to use a specially designed slide rule or a programmable calculator. Slide rules can be usually obtained from film manufacturers. For the mathematically-minded it is a relatively simple task to program a calculator such that it can be used to predict gamma ray exposures.


8-28


Section 9 Sensitiviy

9

Sensitivity

9.1

Radiographic sensitivity Radiographic sensitivity is the ability of radiographic system to reveal discontinuity of certain size on the radiographic image. It can also be defined as a measure of quality of radiographic image. True radiographic sensitivity is difficult quantity to measure.

9.2

Controlling radiographic quality Prior to interpretation of a radiograph it is necessary to establish adequacy of the radiographic technique used. National codes and standards describe devices known as image quality indicators (IQIs). Occasionally the word penetrameter is used when referring to the IQI. It is very important to realise IQI sensitivity is not a direct measure of radiographic sensitivity per se. Good IQI sensitivity does not necessarily indicate good radiographic sensitivity, but it does to some extent prove the quality of the radiographic technique in a general sense. These days the type of IQI most commonly in use is the wire type but other types exist, two examples being the plaque and the step hole types.

9.3

EN ISO 19232-1 wire type IQIs EN ISO 19232-1 wire type IQIs each consist of 7 wires from a list of 19 wires. Four standard wire groupings are available, designation 1, wires 1 to 7, designation 6, wires 6-12, designation 10, wires 10-16 and designation 13, wires 13-19. Each of these groupings is available in any of 4 types of material; steel, designated FE, copper CU, aluminium, AL and titanium TI.

XX= Designation Of Material group (Fe, Cu, Al or Ti)

XX= Designation Of thickest wire (1, 6, 10 or 13)

7 consequently Designated wires (1-7, 6-12, 10-16 or 13-19)

Figure 9.1 EN ISO 19232-1 wire type IQIs.

NDT20-71015 Sensitivity

9-1


Table 9.1 EN ISO 19232-1 wire diameters. Designation W1 W2 W3 W4 W5 W6 W7 W8 W9 W10 W11 W12 W13 W14 W15 W16 W17 W18 W19

Diameter [mm] 3.2 2.5 2.0 1.6 1.25 1.0 0.8 0.63 0.5 0.4 0.32 0.25 0.2 0.16 0.125 0.1 0.08 0.063 0.05

Note: It is fairly easy to remember the wire diameters: if you can remember the diameters of the first three, 3.2, 2.5 and 2mm you can arrive at all other wire diameters by halving as shown below in Figure 9.2. W1=3.2 W2=2.5 W3=2.0

W4=1.6 W5-1.25 W6=1.0

W7=0.8 W8=0.63 W9=0.5

W10=0.4 W11=0.32 W12=0.25

W13=0.2 W14=0.16 W15=0.125

W16=0.1 W17=0.08 W18=0.063

W19=0.05

Figure 9.2 Remembering the EN ISO 19232-1 wire diameters.

Looking along each row the wire diameters are successively halved, eg 3.2, 1.6, 0.8. The EN ISO 19232-1 material groupings are as follows: the Fe designated IQIs (made from low alloy steel) cover all ferrous materials; the Cu (made from copper) cover copper, tin, zinc and their alloys; the Al (made from aluminium) cover aluminium and its alloys; the Ti (made from titanium) cover titanium and its alloys. Special IQIs can be used for materials lying outside these four groups, or the contracting parties could agree to use one of the four normal designations. 9.4

Other wire type IQIs In industry are commonly used IQIs acc. to ASTM E 747. (See Table 9.2) Table 9.2 ASTM E 747 wire diameters. IQI designation A B C D


Wire diameters [mm] 0.08 0.1 0.25 0.33 0.81 1.02 2.5 3.2

9-2

0.13 0.4 1.27 4.06

0.16 0.51 1.6 5.1

0.2 0.64 2.03 6.4

0.25 0.81 2.5 8


9.5

EN ISO 19232-2 step-hole type IQIs BS EN 19232-2 IQIs consist of stepped blocks of material with each step having one or a pair of through drilled holes. Step thicknesses of 0.8 mm or less have two drilled holes, while the thicker steps have a single hole. In each case the step thickness and the hole diameter are equal.

Figure 9.3 EN 19232-2 step-hole IQIs.

These IQIs are supplied encased in plastic complete with lead number identification similar to that used in EN ISO 19232-1 wire type IQIs. Table 9.3 EN ISO 19232-2 step-hole IQIs IQI designation H1 X X X X X X


H5

X X X X X X

H9

X X X X X X

Hole/step number

Hole diameter/ step thickness, mm

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16 H17 H18

0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0 1.25 1.6 2.0 2.5 3.2 4.0 5.0 6.3

H13

X X X X X X

9-3


9.6

ASTM E 1025 plaque type (flat-hole type) penetrameters ASTM E 1025 describes plaque type penetrameters (penetrameter is just another word meaning IQI).

XX= IQI thickness thousandths of an inch

Figure 9.4 ASTM E 1025 IQIs.

Selection of appropriate flat-hole IQI is based on its thickness – usually it is required that plaque thickness shall be equall to 2 % of thickness under test. In order to allow testing of various thicknesses whole set of flat hole type IQIs is availeable. Thickness of particular IQI is expressed in thousands of inch and is indicated by lead ID number (XX). This type of IQI has usually three holes The “1T” hole has a diameter equal to the plaque thickness, 2T two times and 4T four times. Penetrameters up to 160 thousandths of an inch thick are rectangular and contain 1T, 2T and 4T holes. Thicker penetrameters are circular and contain 1T and 2T holes, and hole 4T is substituted by rounded outline of IQI. When using this type of IQI the image quality level is typically specified as 2-2T ocationally also as 1-2T or perhaps 2-4T. If image quality 2-2T is required, than IQI plaque 2 % is required (first digit 2) and on image of IQI shall be visible hole 2T. A total of eight material groups are identified by adding notches to the edges of the penetrameter. Where the component is a weld the reinforcement should be taken into consideration when choosing the IQI. 9.7

IQI sensitivity IQI sensitivity is usually defined as the thickness of the thinnest wire, plaque or step which is visible on the radiograph expressed as a percentage of the specimen thickness. Exactly what is meant by specimen thickness varies from standard to standard and from technique to technique. EN ISO 17636-1 contains tables of essential wires for Class A and B techniques for IQI placed source or film side. ASME V article 2 also permits the use of wire type IQIs and takes a similar essential wire approach. It used to be common good practice to place the IQI in the least favourable position within the diagnostic area of the radiograph. This would usually have meant placing the IQI on the source side of the specimen and towards the extremities of the diagnostic area because this is where the contrast and definition would tend to be at their least favourable (highest value of geometric unsharpness and lowest film density).


9-4


Nowadays, when performing radiography of a weld in accordance with EN ISO 17636-1 the IQI shall be placed preferably source side, possibly film side, in an area of uniform optical density. This usually means on the parent material and at the centre of the area of interest. The wires may or may not be visible in the image of the weld for double wall single image (DWSI) or single wall single image (SWSI) radiography but they shall be placed at 90 to the weld axis and at least a 10mm length of wire shall appear on the parent material in an area of uniform film density. The IQI shall be placed with its wires parallel to the weld axis on the parent material adjacent to the weld. The requirement for visible wire length remains unchanged. In the past it was not uncommon for national codes or standards to specify an overall requirement for a radiographic sensitivity of 2 % or better. This was easy to achieve on thicker sections but often impossible to achieve on thinner sections of material. Modern radiographic standards take account of the fact that the best achievable sensitivity for a given situation and technique is not a fixed quantity but a variable which depends upon such factors as the type of radiation source, the technique and the thickness of the specimen. Such standards specify a minimum sensitivity which should be achievable using a good quality radiographic technique. One such standard is EN ISO 17636-1. Table 9.4 below gives some EN ISO 17636-1 requirements for SWSI radiography with source side IQI placement, DWSI radiography with film side IQI placement and DWDI radiography with source side IQI placement.


9-5


Table 9.4 EN ISO 17636-1 sensitivity requirements for wire type IQIs. Single wall, single image - IQI on source side Image quality class A Image quality class B Nominal thickness/mm IQI Value Nominal thickness/mm to 1.2 W18 to 1.5 above 1.2 to 2.0 W17 above 1.5 to 2.5 above 2.0 to 3.5 W16 above 2.5 to 4.0 above 3.5 to 5.0 W15 above 4.0 to 6.0 above 5.0 to 7.0 W14 above 6.0 to 8.0 above 7.0 to 10 W13 above 8.0 to 12 above 10 to 15 W12 above 12 to 20 above 15 to 25 W11 above 20 to 30 above 25 to 32 W10 above 30 to 35 above 32 to 40 W9 above 35 to 45 above 40 to 55 W8 above 45 to 65 above 55 to 85 W7 above 65 to 120 above 85 to 150 W6 above 120 to 200 above 150 to 250 W5 above 200 to 350 above 250 W4 above 350 Double wall, double image - IQI on source side Image quality class A Image quality class B Penetrated thickness/mm IQI Value Penetrated thickness/mm to 1.2 W18 to 1.5 above 1.2 to 2.0 W17 above 1.5 to 2.5 above 2.0 to 3.5 W16 above 2.5 to 4.0 above 3.5 to 5.0 W15 above 4.0 to 6.0 above 5.0 to 7.0 W14 above 6.0 to 8.0 above 7.0 to 12 W13 above 8.0 to 15 above 12 to 18 W12 above 15 to 25 above 18 to 30 W11 above 25 to 38 above 30 to 40 W10 above 38 to 45 above 40 to 50 W9 above 45 to 55 above 50 to 60 W8 above 55 to 70 above 60 to 85 W7 above 70 to 100 above 85 to 120 W6 above 100 to 170 above 120 to 220 W5 above 170 to 250 above 220 to 380 W4 above 250 above 380 W3 Double wall, single or double image - IQI on film side Image quality class A Image quality class B Penetrated thickness/mm IQI Value Penetrated thickness/mm to 1.2 W18 to 1.5 above 1.2 to 2.0 W17 above 1.5 to 2.5 above 2.0 to 3.5 W16 above 2.5 to 4.0 above 3.5 to 5.0 W15 above 4.0 to 6.0 above 5.0 to 10 W14 above 6.0 to 12 above 10 to 15 W13 above 12 to 18 above 15 to 22 W12 above 18 to 30 above 22 to 38 W11 above 30 to 45 above 38 to 48 W10 above 45 to 55 above 48 to 60 W9 above 55 to 70 above 60 to 85 W8 above 70 to 100 above 85 to 125 W7 above 100 to 180 above 125 to 225 W6 above 180 to 300 above 225 to 375 W5 above 300 above


375

IQI Value W19 W18 W17 W16 W15 W14 W13 W12 W11 W10 W9 W8 W7 W6 W5

IQI Value W19 W18 W17 W16 W15 W14 W13 W12 W11 W10 W9 W8 W7 W6 W5

IQI Value W19 W18 W17 W16 W15 W14 W13 W12 W11 W10 W9 W8 W7 W6

W4

9-6


9.8

Duplex wire IQI Duplex wire type IQI as specified by EN ISO 19232-5 consists of set of 13 element (duplexes) placed in rigid plastic holder. It is used to assess total image unsharpness of radiographic image. Each element consist of pair of wires with equall circular cross-section. Its separation equals to wire diameter. Duplexes D1, D2 and D3 are made of tungsten, the rest of duplexes is made of platinum. The largest element in which the image of wire has just merged without an identifiable soace between the images of two wires defines the limit of perceptibility The total imabe usnharpness is twice the diameter of wire in the element and diameter is considered to be a spatial resoluteion of image. Table 9.5 Duplex wire type IQI - Unsharpness and basic spatial resolution readout: [mm]

Achieved basic spatial resolution ܴܵ௕ [mm]

Achieved geometrical unsharpness ܷ௚ [mm]

D1

0,80

0,80

1,60

D2

0,63

0,63

1,26

D3

0,50

0,50

1,00

D4

0,40

0,40

0,80

D5

0,32

0,32

0,64

D6

0,25

0,25

0,50

D7

0,20

0,20

0,40

D8

0,16

0,16

0,32

D9

0,13

0,13

0,26

D10

0,10

0,10

0,20

D11

0,08

0,08

0,16

D12

0,063

0,063

0,13

D13

0,05

0,05

0,10

Duplex identification


Wire diameter

9-7


Section 10 Radiographic Techniques (For Welds in Plate and Pipe)

10

Radiographic Techniques (for Welds in Plate and Pipe) Three basic techniques are used for the radiography of butt welds in pipe, these being the single wall single image (SWSI), double wall single image (DWSI) and double wall double image DWDI techniques. All radiographs of butt welds in plate will, in general, be SWSI. Each technique is discussed below, paying particular attention to the extent of the diagnostic area, the minimum source to film distance, the placement of location markers and IQIs.

10.1

IQI type and placement It is important that IQIs are placed source or film side and at a position within the diagnostic film length (DFL) in accordance with the requirements of the contract specification. As a general rule, wherever possible, the IQI should be placed source side. As then they are affected both by radiographic contrast and geometric unsharpness. Film side IQIs indicate radiographic contrast only, thus source side IQIs give a more accurate measure of the overall radiographic quality. It used to be standard good practice to place wire type IQIs towards the end of the diagnostic area, with the thinner wires toward the outside of the DFL; the wires were invariably placed across the weld and sensitivity was assessed on the weld allowing for any weld reinforcement present. This way of working would still meet ASME V article 2 requirements, although this document does not specify where within the DFL the IQI should be placed. In Europe matters are different; when working in accordance with EN ISO 17636-1 sensitivity should generally be assessed at the centre of the DFL on the parent material. Plaque and step hole type IQIs should, preferably, always be placed at the centre of the diagnostic area on the parent material. Should the image of these IQI types encroach on the weld area the radiograph should be re-taken. If working with a wire type IQI in accordance with ASME V/2 sensitivity would probably be measured on the weld. ASME V article 2 then has a requirement that the film density through the diagnostic length shall not vary by more than +30 or -15% from that measured at the IQI. The same allowable density variation applies to plaque type IQIs, but these, of course, must be placed alongside, not on the weld. Plaque type IQIs may be shimmed to compensate for any weld reinforcement. If a technique produces a wide range of film density the placement of several IQIs may be necessary in order to meet the allowable density variation requirement. EN ISO 17636-1 limits the diagnostic film length (DFL) by specifying that the penetrated thickness at the ends of the DFL shall not exceed 110% (Class B techniques) or 120% (Class A techniques) of the thickness penetrated at the centre of the DFL.

NDT20-71015 Radiographic Techniques (For Weld in Plate and Pipe)

10-1


10.2

Location markers All national codes and standards require the use of location markers, usually in the form of lead letters or numbers that appear in the radiograph as a radiographic image. It is very important that the markers are placed in such a way as to prove coverage of the weld where a multiple exposure technique is used. Three general rules apply: 1 2 3

When performing radiography of welds in flat plate location markers must be placed source side. Film side markers will not prove coverage because of parallax. When performing radiography of welds in curved surfaces location markers should be placed on the convex surface for all techniques where the source or focus to film distance is equal to or exceeds the radius of curvature. When performing radiography of welds in curved surfaces location markers should be placed source side for all techniques where the source or focus to film distance is less than the radius of curvature.

Figure 10.1 Location marker placement – parallax effect on flat plate.


10-2


10.3

Identification of radiographs All national codes and standards require unique and permanent identification of radiographs. In general this can be applied by any suitable means although in some cases identification using lead numbers that appear as radiographic images is required. Where not prohibited by the contract specification-flashing the radiographic identification is a good method. The required identification is written on a scrap of white paper, the radiograph is suitably masked and the scrap of paper is placed on the unmasked area. The radiograph is then flashed with a suitable light source and the identification becomes visible during subsequent film processing. Exactly what constitutes an acceptable unique identification varies widely from specification to specification, but the minimum is a unique number. ASME V article 2 requires a unique weld number, the date and the manufacturer’s name or symbol. Most codes require radiographs of repair welds to be marked with R1, R2 and R3 etcetera depending on the number of repair attempts. RW is commonly used to identify a complete reweld. Items such as heat treatment condition, welder number and welding procedure reference may also be required.

10.4

Radiation energy EN ISO 17636-1 specifies the maximum X-ray tube voltage which may be used based on the component thickness. EN ISO 17636-1 also specifies the minimum and maximum thickness on which each type of gamma ray isotope may be used (see Table 10.1 below). Table 10.1 EN ISO 17636-1 applicable thickness ranges for gamma ray sources and high energy X-rays. Penetrated thickness, w, mm Radiation source Class A Class B w5 w5 Thulium 170 1  w  15 2  w  12 Ytterbium 169 (1) 10  w  40 14  w  40 Selenium 75 (2) 20  w  100 20  w  90 Iridium 192 40  w  200 60  w  150 Cobalt 60 30  w  200 50  w  180 X-ray equipment, 1-4MeV w  50 w  80 X-ray equipment, 4-12MeV w  80 w  100 X-ray equipment, 12MeV and above (1) For aluminium and titanium, the penetrated material thickness is 10  w  70 for Class A and 25  w  55 for Class B. (2) For aluminium and titanium, the penetrated material thickness is 35  w  120 for Class A.

ASME V/2 requires that the radiation energy employed for any radiographic technique shall achieve the density and IQI image requirements… Standard ASTM E 94 may provide some additional guidance.


10-3


10.5

Source to film distance The minimum source to film distance for EN ISO 17636-1 is calculated using the formula: ∙ ∙√ where is the minimal required source to object distance, is the effective source or focus size, is the object to film distance and is a constant equal to 7.5 for Class A techniques and 15 for Class B. EN ISO 17636-1 also includes a nomogram for the less mathematically minded. ASME V/2 limits the minimum source or focus to film distance by specifying maximum geometric unsharpness, 0.51mm for component thickness up to 50.8 mm, 0.76 mm for greater than 50.8 and up to 76.2 mm, 1.0 mm for greater than 76.2 and up to 101.6 mm and 1.78 mm for component thickness exceeding 101.6 mm

10.6

SWSI techniques

10.6.1 SWSI technique for plate

Figure 10.2 EN ISO 17636-1 SWSI technique for flat plate.

Figure 10.2 shows a typical set-up for exposure of a butt weld in flat plate. The annotations refer to EN ISO 17636-1 requirements. The source should be positioned on the centre line of the weld, directly above the centre of the diagnostic area.


10-4


10.6.2 SWSI technique: source internal, placed centrally (panoramic technique)

Figure 10.3 SWSI panoramic technique (EN ISO 17636-1).

Required number of exposures = 1 (see Figure 10.6 for EN ISO 17637-1 requirements). This technique is commonly used for pipeline welds where specially designed, remotely operated, devices known as crawlers are often used. These machines can travel up to several kilometres along the inside of the pipeline in order to reach the desired position to radiograph a particular weld. The typical battery life for an X-ray crawler will usually allow about 100 exposures to be made between successive battery charges. Gamma ray crawlers are also used. This technique may also be used for examining girth welds in cylindrical pressure vessels. Using thulium 170 isotopes boiler tube welds, which may have an outside diameter of only 40mm, are occasionally examined by this technique. The major advantage of this technique is that it can radiograph an entire girth weld in a single exposure. With this technique location marker placement is not critical, but it is usually more convenient to place the markers film side. In most cases it will be impractical to place the IQI source side for this technique, although source side IQIs would be preferred if access is not a problem, film side IQIs are therefore generally used. Comparator radiographs having IQIs placed source and film side can be used to establish sensitivity requirements for film side IQIs. In most cases three IQIs are placed at 120 intervals around the circumference, although some specifications require more or fewer than this. The radiograph may consist of a number of overlapping films or it may be a single length of rollpack film. Identification of the film may be included as a radiographic image but it may also be added later. Where several overlapping films are used each film must be uniquely and permanently identified.


10-5


10.6.3 SWSI technique: source internal, offset

Figure 10.4 SWSI source internal and offset technique (EN ISO 17636-1).

Required number of exposures: see Figure 10.6 for EN ISO 17636-1 requirements. In some cases it may not be possible to satisfy the requirements of the applicable specification for geometric unsharpness if the panoramic technique is used. Where this is the case it may be possible to achieve a satisfactory geometric unsharpness by offsetting the source towards the inner wall of the pipe. Location markers should be placed film side if the SFD or FFD is longer than the radius of curvature of the test item. If the converse of this is true (as may be the case for a large diameter pressure vessel) then the location markers should be placed source side. 10.6.4 SWSI technique: film inside, source outside

Figure 10.5 SWSI film inside, source outside (EN ISO 17636-1).

Required number of exposures: see Figure 10.6.


10-6


t/De t/De

De/f

De/f De = external diameter of pipe, f = source to object distance, t = nominal wall thickness Figure 10.6 EN ISO 17636-1: Exposures required for film inside source outside (FISO) techniques.


10-7


This is a rather unpopular technique because a large number of exposures (usually 8 or more) are required to cover the entire circumference of the weld. In general it will only be used when an acceptable radiograph cannot be achieved using either of the two single wall techniques described in 10.6.2 and 10.6.3 and can also not be achieved using the double wall techniques described. Location markers must be placed source side. The IQI should always be placed source side, there is no excuse for using a film side IQI when using this technique. Identification of the films may be included as radiographic images (although it will probably be impractical to use long identifications due to the limited amount of area available on the film) but may also be added later. 10.7

Double wall single image Where there is no access to the inside of a pipe double wall techniques have to be employed. In the DWSI technique the source of radiation is usually placed at the minimum possible distance from the film. The reason is that as the source to film distance increases so does the number of exposures needed to cover the entire circumference of the weld. In addition, any improvement in image quality due to the reduced geometric unsharpness associated with an increase in SFD or FFD has to be offset against a reduction in image quality due to increased scatter. Geometric unsharpness limitations permitting gamma sources can be placed almost in contact with the outside surface of the pipe. In many cases this reduces the required number of exposures to just three (see Figure 10.6 for EN ISO 17636-1 requirements). X-ray tubes are bulky and the minimum achievable FFD will usually be about 125 mm plus the outside diameter of the pipe. A minimum of four exposures per weld is therefore required when using an X-ray source for this technique. Being able to place the source of radiation in close contact with the pipe gives gamma ray another significant advantage over X-ray techniques particularly on smaller pipe diameters. Less offset is needed with gamma ray sources in order to ensure that the image of the source side portion of the weld is not superimposed upon the film side part of the weld. This can increase the chance of finding vertical defects such as lack of root fusion in the weld being radiographed. As the wall thickness to diameter ratio increases the DWSI technique becomes increasingly difficult to apply, the number of exposures required increases and the quality of the radiographs produced diminishes. For these reasons DWSI (superimposed) techniques tend to be preferred for heavy wall small diameter pipes. Because there will in general be no access to the inside of the pipe when this technique is employed the location markers and IQI are always placed film side.


10-8


Figure 10.7 Double wall single image technique.

Required number of exposures: see Figure 10.8.


10-9


De = external diameter, t = nominal thickness, SFD = source to film distance Figure 10.8 EN ISO 17636-1: Exposures required for DWSI and SWSI source inside film outside techniques.


10-10


10.7.1 Double wall double image (elliptical)

Figure 10.9 DWDI technique (elliptical).


10-11


In accordance with EN ISO 17636-1 this technique is limited to girth welds in pipe having an OD of less than 100mm. In accordance with ASME V article 2 welds in pipe of up to 3½ inch nominal diameter (OD about 88.9mm), may be radiographed using DWDI. For pipes with a wall thickness to OD ratio in excess of about 0.12 the DWDI (superimposed) technique is preferred. The minimum number of exposures required by both EN ISO 17636-1 and American standards is two at 90 to each other. Long source to film distances are needed because the minimum value of object to film distance is equal to the OD of the pipe. Exposure times for this technique, therefore, tend to be rather long especially in the case of gamma ray techniques. A single location marker on each exposure is generally sufficient, although some specifications require pitch markers (A to B, B to C, C to D and D to A, etcetera). Location markers may be placed source side or film side. IQIs should always be placed source side. EN ISO 17636-1 requires wire type IQIs to be placed on the parent material with their wires parallel to the weld axis, see Figure 10.9. Working in accordance with ASME V article 2 standard wire type IQIs should be placed with their wires across the weld at 90 to the weld axis. In the DWSI technique the film is wrapped around the pipe to remain as close as possible to the weld. Conversely, in the DWDI technique the film should be kept as flat as possible, see Figure 10.9. 10.7.2 DWDI (superimposed) This has the same range of application as the elliptical technique, but is preferred when the thickness to OD ratio exceeds 0.12. Welds having difficult geometry that may prevent them from being radiographed using the elliptical technique can generally be radiographed successfully using this technique. As the image of the source side part of the weld is superimposed on the image of the film side part of the weld it is often not possible to accurately locate a weld defect when using this technique. This is not usually much of a handicap because small diameter welds tend to be cut out and re-welded rather than being repaired locally. A single location marker per exposure is usually sufficient when using this technique and it may be placed either source or film side. IQIs should always be placed source side. EN ISO 17636-1 and ASME V article 2 both require a minimum of three exposures at 120 to spacing (or three at 60 spacing for difficult access situations) for this technique. The DWDI superimposed technique may be more likely than the elliptical technique to successfully detect lack of root fusion due to the more favourable angle of incidence of the primary beam.


10-12


Figure 10.10 DWDI technique (superimposed).


10-13


Section 11 Interpretation of Radiographs

11


11.1

Introduction Interpretation of radiographs is a skill only gained through long experience. This section gives the reader a guide to radiographic interpretation and should be regarded as a base upon which to build. The interpretation of a radiograph should not be confused with the acceptance or rejection of a component. The radiograph must first be interpreted and any defects observed assessed against the applicable standard. A weld or casting must be accepted on its merits or rejected for its faults and should neither be accepted nor rejected due to difficulties encountered in the interpretation of radiographs. Any radiograph not meeting code requirements with regard to radiographic quality must be rejected. In circumstances where there is doubt as to the nature of a radiographic image it is often necessary to visually inspect the component or to cross check the radiographic results using another NDT method.

11.2

Viewing conditions The success or failure of radiographic interpretation is highly dependent upon the film viewing conditions. The eye is very sensitive to small variations in film density once it has developed night vision. Anyone carrying out radiographic interpretation should therefore not begin to view radiographs until night vision has developed. Since this cannot be achieved in a brightly lit room it is important that the films are viewed in low ambient light. Night vision takes several minutes to develop and so the films should not be viewed immediately upon entering the viewing room. Five minutes is the recommended period that should elapse before critical interpretations are made. It is also important that film is properly masked on the viewer so that the light falling on the eye comes from the radiograph only. If the film is not adequately masked the eye will be blinded by the bright light coming from around the film. Radiographs are easily damaged, therefore the viewing room must be clean and dry and the radiographs must be handled with care. The viewer should be mounted on a table or bench large enough to allow the films to be spread out without the danger of them falling to the floor. A well shielded reading lamp will allow reports to be read or notes to be made, without unduly increasing the overall ambient lighting. The radiographs should be viewed at a normal reading distance (normally less than 400mm). A low power magnifier (2 or 3X) may occasionally be helpful, but it should not be necessary for routine examination. In accordance with governing standards requirements the visual acuity of the radiographic interpreter must be J1 (corrected or uncorrected) in at least one eye. Additionally a perception of grey shades should be verified. The viewing of radiographs is often undertaken in the dark room where the film was processed. This is satisfactory provided that the viewing bench or table is clean and well away from the processing tanks. Under normal circumstances films should NEVER be viewed whilst wet. There are two reasons for this: 1

The film emulsion is swollen with water and the images are not as clear as when the film is dry.

2

Emulsion is very delicate and any attempt to mask the film will result in scratches or marks on the film, effectively ruining it.

NDT20-71015 Interpretation of Radiographs

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National standards generally require that the illuminance of a radiographic film viewer be sufficient to produce a transmitted light intensity of at least 30 and preferably 300 candela per square metre (cd/m2). This means that a viewer suitable for viewing radiographic film with a density of 3.0 must have an illuminance of at least 30,000 cd/m2 with as much as 300,000 cd/m2 being desirable. BS EN 25580 requirements for radiographic film viewers are given by Table 11.1 below. Note: These are minimum requirements. Table 11.1 BS EN 25580 requirements for radiographic film viewers.

Film density 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

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Minimum screen luminance, cd/m2 300 1,000 3,000 10,000 10,000 30,000 100,000 300,000

Transmitted light luminance, cd/m2 30.0 31.6 30.0 31.6 10.0 9.5 10.0 9.5

Reporting The initial interpretation of a radiograph should always be undertaken by the manufacturer or designated representative. Other interested parties should be presented with a report which includes an interpretation of each film. They check this and agree or disagree with it. The radiographic report should contain the following as a minimum:        

        

Identification of the item radiographed. Date of manufacture. Date of radiography. Exposure details including the type of equipment used and the tube voltage for X-ray and the type of isotope for gamma ray techniques. Type of film used. Type and thickness of the intensifying screens used. Geometric details, particularly the FFD or SFD and the effective focus or source dimension. Details of the component being radiographed, including the type of material and method of manufacture, the thickness, the heat treatment condition and the repair status. Method of film processing. Film density achieved. Radiographic sensitivity achieved. Technician’s name, signature and date. Interpreter’s name, signature and date. An interpretation of each film and a statement of the component’s acceptability or not. Code or standard applicable to the radiographic technique. Acceptance code or standard. Reference to a written procedure or technique sheet.


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11.4

Film quality The success of radiographic interpretation is dependent upon the quality of the film presented. If the film does not meet the minimum applicable standards for quality then it should be rejected and reshot. The manufacturer’s interpreter may, for economic reasons, not be inclined to reject radiographs which do not meet the minimum quality standards. Therefore any third party viewing the radiographs should be extremely careful to correctly assess the quality of the radiographs prior to endorsing the relevant report, otherwise they will be open to criticism should the film become the subject of any subsequent legal inquiry. When assessing a film for quality a number of items must be considered. These are discussed below.

11.4.1 Component identification All radiographs must be permanently and uniquely marked with sufficient information so as to permit their identification with the component radiographed at a later stage. It is often useful to include such items as the date of test and heat treatment or repair status of the component in the identification. Radiographic identification could appear on the radiograph as a radiographic image but there is usually no reason why it should not be added by any other suitable means. A written procedure should be in force describing the standard method to be used for identifying radiographs. 11.4.2 Location markers Location markers on a radiograph serve two functions: they permit the radiograph to be identified with the area of the component radiographed and they serve to prove that the component has been fully covered by the technique used. Refer to the sections above on radiographic techniques for details. Wherever possible location markers should permanently identify the radiograph with the area radiographed. Items such as pressure vessels are usually hard stamped with a permanent radiographic datum. A written procedure should be in force which describes the standard method used for the placement of location markers. 11.4.3 Film density It is important that the film density is within the specified range since a film having low film density will also have inferior film contrast. EN ISO 17636-1 requires a minimum film density of 2.0 for Class A radiography and a minimum of 2.3 for Class B. ASME V article 2 requires a minimum of 1.8 for X-ray and 2 for gamma ray techniques. In most cases (including EN ISO 17636-1 and ASME V article 2) the minimum figures for film density apply to the area of interest (the diagnostic area) on the radiograph. In weld radiography, for example, film density should generally be measured on the weld area between the location markers (which identify the ends of the diagnostic film length). Density is usually assessed by using a measuring device known as a densitometer. Anyone accepting radiographs which do not meet the applicable density requirements is open to criticism at a later stage should litigation follow a component failure.


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ASME V article 2 requires that the film density within the area of interest must not vary by more than minus 15 or plus 30% from the value measured through the body of the IQI. If necessary additional IQIs can be used in order to satisfy this requirement for exceptional areas. Occasionally an upper limit is specified for film density. ASME V article 2, for example, specifies an upper limit of 4. 11.4.4 Radiographic sensitivity Radiographic sensitivity is not directly related to the minimum detectable defect size. However, a radiograph that meets the applicable code requirement for radiographic sensitivity is much more likely to provide good defect sensitivity than a radiograph which fails to meet the code requirements. The sensitivity of a radiograph depends upon the parameters chosen to produce that radiograph (see the section on the production of a radiograph). If any of the relevant parameters are altered the sensitivity will be affected. It is therefore essential to use IQIs to prove that adequate radiographic quality has been attained. Except in the case of the panoramic technique, which has been described above, at least one IQI should generally appear on each radiograph. Anyone viewing radiographs should be careful to check the radiographic sensitivity meets the requirements of the applicable code. Anyone who fails to do is open to criticism should litigation follow a component failure. 11.4.5 Artefacts and other unwanted images In film radiography an artefact can be defined as any image resulting from a cause that is not directly associated with the object that has been radiographed. Artefacts can be produced by mechanical or chemical damage to the film and by damaged or dirty intensifying screens. Sometimes radiographic images may be formed by things such as debris on the inside of a pipe. These images, while they are strictly speaking not artefacts, can also interfere with the proper interpretation of the radiograph. When radiographs are produced on a commercial basis it is not possible for every film to be free from artefacts. An artefact only becomes significant when it cannot be identified as such or when it hinders the interpretation of the film. These two factors are rather subjective but if any doubt exists then the interpreter should call for a repeat radiograph. A list of possible artefacts is given in the next section. 11.5

Interpretation of radiographic images Three types of image may appear on a radiograph, due to: 1 2 3

Artefacts. Surface irregularities in the component. Internal discontinuities in the component.

Every image within the diagnostic area of a radiograph must be identified as one of these three. It is not permissible to reject a component simply because an image appearing within the diagnostic area cannot be interpreted or because artefacts which are not within the diagnostic area. The following sections attempt to give a description of various types of image which may be seen. The ability to successfully identify all flaws and artefacts on a radiographic images is a skill which can only be perfected with time and experience.


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11.6

Artefacts Pressure marks (crimp marks) Produced by careless film handling - if the film is crimped or buckled either before or after exposure crescent-shaped images in the processed radiograph will result. Light marks indicate crimping before exposure, dark marks crimping after exposure but before film processing. It is usually possible to identify crimp marks by viewing the film in reflected light. They should appear as indentations in the surface of the film. Lead screens which have been crimped should be discarded. Scratches: On the film Radiographic film emulsion is delicate; it is easily damaged if handled carelessly at any stage. Areas used for film handling must be free from dust and films must be handled carefully at all times. Depending upon how severe and when or how formed film scratches may produce either light or dark images, and can usually be identified using reflected light. Scratches: On lead intensifying screens May appear as either light or dark images which cannot be seen in reflected light. If the intensifying screens used to make the radiograph can be positively identified then it may be possible to trace the shape and position of such an image to a scratch on the screens. Even so it will probably be necessary to reshoot the radiograph. Scratched lead screens should be discarded. Dirt: On the film or screens Dirt which finds its way between the film and the screens will generally produce a light image on the resultant radiograph not visible in reflected light. Greasy fingers will produce dark marks on a finished radiograph which can easily be seen in reflected light, but light marks before development. Streakiness or mottling: Poor development Usually caused by insufficient agitation in the early stages of development and is due to a process known as bromide streaming. Reaction products from the chemical interaction of the developer with the silver halides in the film emulsion tend to build up around high film density zones. These reaction products slow down the action of the developer, and since they are relatively heavy they tend to flow down the surface of the film leading to a light coloured streak in the finished radiograph. Under- or over-development usually leads to a mottled effect on the finished radiograph. A similar effect will be produced by developer which has passed its service life. In less severe cases such artefacts may not be a cause for rejection of the radiograph but darkroom procedures should be reviewed to prevent a recurrence or a further deterioration in radiographic quality. Developer splashes Appear as dark spots on the film and indicate poor dark room practice and are usually visible in reflected light. Fixer splashes Appear as light spots on the film and again indicate poor dark room practice. Such marks are usually visible in reflected light.


11-5


Water splashes Appear as either light or dark images on a radiograph. Water splashes before exposure tend to cause light marks, after exposure tend to cause dark marks. Such marks are usually visible in reflected light. Water marks Easily seen on the radiograph in both transmitted and reflected light and are due to uneven drying. They commonly occur where a dry or partially dry film is wetted locally either by splashing or by excess water running down from a film clip. The appearance of water marks can be reduced or eliminated by using a squeegee to remove excess water or a final wash that contains a small amount of detergent (ie a wetting agent). Air bells Light marks caused by air bubbles adhering to the film in the early stages of development and will not occur if the film is properly agitated. Diffraction mottling Can be a problem when X-rays are used to radiograph large grained material, for example being austenitic steels. Diffraction is an apparent bending of a beam of radiation due to interference. It occurs when radiation passes through a grating that has spacing approximately equal to one wavelength. The spacing of atoms in a metallic crystal is about 0.1nanometres. This corresponds to X-ray radiation with photon energy in the region of 10keV. If low energy components are removed from the X-ray beam by filtration the problem with diffraction mottling will disappear. Diffraction mottling does not occur in gamma radiography because of the absence of low energy beam components. Diffraction can be used to advantage and it is the basis for the study of metal crystals by X-ray crystallography. Static marks Penetrating radiation is by definition ionising. It always causes the build-up of an electric charge on the film during exposure but under normal circumstances this is not a problem because the charge quickly flows to earth. In dry climates, however, a static charge may remain on the film until it is unloaded in the darkroom, whereupon it flows to earth suddenly in a manner which could be painful for the radiographer. Such a sudden dissipation of electrical energy leads to the emission of a sudden burst of light which produces dark tree-like marks on the finished radiograph. Static marks can be avoided by careful film handling. Dichroic fogging Radiographs affected by dichroic fog will appear reddish when viewed using transmitted light and greenish in reflected light. Dichroic means two-coloured. It is caused when the development process continues during the fixing process, when the fixer solution has become insufficiently acidic to stop the development process. The use of an acidic stop bath between the development and fixing processes will generally prevent the occurrence of this seldom seen artefact.


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Reticulation Appears on the radiograph as an orange peel-like mottling effect. It is caused when the film emulsion is subjected to a temperature shock at any stage during the film processing as the sudden change in temperature causes the film emulsion to wrinkle. It will not generally occur as long as the sudden change in temperature is less than 10°C. Film fogging by X- or gamma rays If radiographic film is not stored well away from sources of ionising radiation then it is likely to become fogged. Films which have been fogged in this way will produce reduced radiographic contrast (fogging has much the same effect as scattered radiation which is explained in a section above). If it is suspected that the film is fogged then the level can be checked by processing a piece of unexposed film. Film which has a density due to fogging of 0.3 or more is not suitable for use in high quality industrial radiography. Light fogging Exposure to light other than from darkroom safe lamps (and even prolonged exposure to safe lamps) will cause fogging at any stage prior to fixing the film. Such fogging may be localised or general - localised is not a problem unless it encroaches onto the diagnostic film area but general light has the same effect as fogging due to exposure to ionising radiation. Film fogging due to inadequate storage conditions Film stored at too high a temperature or exposed to chemical fumes may become fogged. The fog level of all film increases with age, even under ideal storage conditions, therefore all film boxes are marked with an expiry date. High speed films deteriorate more quickly than slower ones. Solarisation Image reversal due to extreme over exposure to X or gamma rays or caused by exposure to light during film development. A final word on artefacts It should be stressed again that artefacts are cause for rejecting the film only if they interfere with interpretation. A large number of artefacts present on radiographs indicate poor practice and the interpreter should take time to inspect the radiographic facilities and review darkroom procedures. 11.7

Interpretation of weld radiographs Radiographic indications due to surface geometry It is usually possible to successfully interpret weld radiographs in the as-welded condition. Experience will help the interpreter identify surface marks which are normal for a particular welding process and technique. Where there is doubt if a visual examination of the weld will often help. It is felt that an indication resulting from surface geometry could mask a significant defect indication, or where visual examination proves inconclusive, it may be necessary to dress the weld to a smooth contour and reshoot the radiograph. The severity of weld defects such as excessive penetration or undercutting is difficult to judge using radiographic evidence alone, wherever possible defects of this type should be judged by visual means.


11-7


Listed below are some of the common surface conditions that can produce radiographic images. Excessive root penetration Excess weld material protruding through the root of a single sided fusion weld. It appears in the radiograph as a continuous or intermittent light irregular band within the image of the weld. Common causes of excessive penetration are, no root face, root gap too wide, excessive amperage, travel speed too slow and incorrect polarity.

Figure 11.1 Excessive root penetration.

Root concavity A shallow groove which may occur in the root of a single sided weld, it appears in the radiograph as a series of dark areas along the centre of the weld varying in density according to the depth of imperfection and it is often seen in welds made with the use of a backing gas. The pressure of the backing gas can cause the weld root to collapse during welding of the first subsequent weld run (hotpass). Other possible causes are no root face; travel speed too slow, amperage too high, incorrect polarity on the hot pass; excessive pre-heat and root gap too narrow.

Figure 11.2 Root concavity.

Incompletely filled groove (lack of fill) A continuous or intermittent channel along the edge of the weld due to insufficient weld material. It is a fusion defect and should not be confused with lack of reinforcement or undercutting. It produces an image in the radiograph of a straight edged (on one side at least) dark band and is caused by poor welding practice.

Figure 11.3 Incompletely filled groove or lack of fill.


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Lack of reinforcement This is a concave area of the weld cap where the weld is locally thinner, sometimes thinner than the parent material. In the radiograph it appears as a dark area towards the centre of the weld which has diffuse edges and is caused by poor welding practice.

Figure 11.4 Lack of reinforcement.

Undercut An irregular groove at the toe the weld in the parent material due to burning away during welding. It appears in the radiograph as a dark/irregular/ intermittent band in a position adjacent to either the cap or root weld toe or between adjacent capping runs. It may therefore appear inside or outside the weld image on the radiograph. The major causes are excessive amperage and poor welding technique. Welds in the vertical or horizontal – vertical position tend to be prone to undercutting.

Figure 11.5 Undercut.

Spatter Spatter consists of globules of molten filler metal expelled during arc welding on to the surface of the parent material or weld. It appears in the radiograph as small light spots and the major causes of spatter are incorrect polarity and welding current too high. Spatter particularly affects MIG, MAG, MMA and FCAW, and is highly unlikely to be seen in association with welds made by TIG or SAW. In pipe welding spatter is possible on both external and internal surfaces.


11-9


Figure 11.6 Weld spatter.

Excessive dressing/grinding marks A reduction in material thickness caused by the removal of the surface of a weld and adjacent areas to below the surface of the parent material. Excessive dressing appears as a dark area with diffuse edges, whilst a grinding mark appears as a dark area that will usually have clearly defined edges, caused by poor practice or access for welding.

Figure 11.7 Excessive dressing.

Hammer marks (tool marks) Indentations in the surface of the parent material or the weld resulting from the application of a tool, for example a chipping hammer. They mostly appear in a radiograph as dark half-moon shaped areas usually having clearly defined edges. They are caused by poor fabrication practice and often result from attempts to correct welding distortion. Torn surface A surface irregularity due to breaking off of temporary attachments. The radiographic indication produced has a shape corresponding to that of the affected area which may be either light or dark depending on whether part of the attachment has remained or parent material has been torn away, caused by poor fabrication practice, often seen in association with storage tank or ship hull welds. Surface pitting Surface imperfection, usually of the parent material but also the weld metal where a component has been in service. It usually takes the form of small depressions resulting from localised corrosion. Pitting appears in a radiograph as small dark rounded images and it is possible to mistake this for a welding defect, as its appearance can be identical to that of porosity.


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Internal defects:Cracks In weld radiography four basic types of crack are sometimes detected by radiography. 1 2 3 4

Centreline (shrinkage). Transverse (including chevron). Heat affected zone or toe. Crater.

A crack is a linear discontinuity produced by a fracture. In welding, cracks can occur after the completion of welding, during the deposition of subsequent welding runs or at the point of solidification and can affect both the weld deposit and the parent material. Cracks are often invisible on radiographs but if detected appear as dark, fine often branching lines, usually diffuse or discontinuous. The ability of the radiographic technique to detect a crack is dependent on the crack’s orientation relative to the direction of the radiation. Figure 11.8 shows how even a slight deviation from the optimum orientation will greatly reduce the change in section thickness which the radiation experiences due to a planar defect such as a crack. In the case shown a variation from optimum incidence of just 1 will reduce the change in penetrated thickness from 10 to 1mm for a planar defect measuring 10mm by 17μm.

Figure 11.8 Detectability of planar defects.

Centreline cracks (also called shrinkage or solidification cracks) Centreline cracks are caused by excessive restraint or the deposition of too much weld metal in a single pass, from excessive amperage or travel speed too slow and are possible for all arc welding methods. They occur at the point of solidification when the weld metal has a very low tensile strength and are the welding equivalent of a hot tear. Centreline cracks are probably the easiest to detect by radiography, as they tend to be much wider than other types as their detectability is less strongly affected by changes in the direction of the primary beam. Transverse and chevron cracks Any crack that lies across the weld axis is called a transverse crack. There are two distinct types and usually occur when the compressive strength of the parent material is significantly greater than the tensile strength of the weld metal. NDT20-71015 Interpretation of Radiographs

11-11


A shrinkage or solidification crack which usually occur at 90 to the weld axis, often affecting the root pass of single sided welds. In nature they are very similar to centreline cracks, but the source of restraint is different and are relatively easy to detect by radiography. The second type is a chevron crack, which occurs at about 45 to the weld axis, usually after the completion of welding. Chevron cracks are a special type of hydrogen induced crack; the stress that causes the crack being due to an excessive amount of dissolved hydrogen in the weld metal. They are sometimes detected by radiography, but where there is a known problem, other NDT methods with a higher probability of detection should be used. Heat affected zone cracks and toe cracks Various mechanisms can lead to cracking in the heat affected zone (HAZ) of a weld. HAZ cracks often start at or run to the toe of the weld since there is always a high stress concentration at this point. In ferrous welds the hardest, most martensitic, brittlest microstructure is usually found in the HAZ. It is this susceptible grain structure that makes the HAZ a prime site for cracking. HAZ cracks are usually caused by one of two mechanisms. The first involves dissolved hydrogen. Molten iron has a very high solubility for hydrogen while solid iron has a very low solubility. Thus as the metal freezes hydrogen will attempt to leave solution and escape from the weld pool but this process is slow compared with freezing, therefore most of the hydrogen becomes trapped in the solidified metal. The trapped hydrogen then diffuses through the metal crystals and begins to build up an internal pressure at points of weakness, usually the grain boundaries. In some cases the internal pressure exceeds the strength of the material and hydrogen cracking occurs. Hydrogen induced cracking may occur up to 48 hours after welding. Where ferrous materials operate in a hydrogen rich environment, for example in sour gas service, hydrogen cracking can occur as an in-service defect. High strength, high carbon equivalent steels are most prone to hydrogen cracking. The presence of trace elements, especially sulphur and phosphorus can make hydrogen cracking much more likely to occur. Hydrogen induced cracks are not likely to be detected by radiography and other methods such as ultrasonic testing should be used in any situation where there is a high probability of occurrence. A second type of cracking that can occur in the HAZ of a weld is sometimes called weld decay. This can affect stainless steels and is caused by the precipitation of brittle material (chromium carbide) at the grain boundaries. All stainless steels contain a small proportion of carbon which is generally held in solution within the austenitic grains, but the heat from welding can cause it to combine with the chromium which is present forming chromium carbide which is an extremely brittle material. Weld decay can be avoided by reducing the carbon content of the parent material and filler wire. Cracking caused by weld decay is unlikely to be detected by radiography. Crater cracks Occurs when the heat source is removed too suddenly at the end of a weld run. The cracking mechanism is the same as that for centreline cracking. The major dimension of a crater crack is usually less than 5mm, are often star shaped in a radiograph and relatively easy to detect. Some standards will permit this type of cracking provided that it does not exceed a specified maximum dimension.


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Figure 11.9 Typical radiographic appearance of a crack.

Lack of fusion Can occur either between the weld deposit and the parent material or between successive layers of weld material, also due to lack of fill or penetration. Lack of fusion is where the solid material immediately adjacent to the molten weld pool failed to become molten during the welding process leading to a lack of union between the molten weld material and the adjacent solid material. The ability of radiographic techniques to successfully detect lack of fusion is strongly dependent on the orientation of the defect with respect to the incident beam of radiation, (see Figure 11.8). Given favourable orientation lack of fusion with the parent material will appear in the radiograph as a fine dark straight line which may be continuous or intermittent. Unfavourably orientated lack of fusion with the parent material may sometimes still be detected due to the presence of associated slag inclusions or porosity. A slag inclusion with a straight edge normally indicates lack of fusion and gas escaping from an area lack of fusion during the deposition of a subsequent welding run may lead to a line of linear porosity. Lack of fusion between subsequent layers of weld material will generally not be detected by radiography unless it is associated with another type of defect such as slag.


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Figure 11.10 Types of lack of fusion.


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Figure 11.11 Lack of fusion in the radiograph.


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Incomplete root penetration The failure of the weld material to extend into the root of a joint and is a fusion defect, not to be confused with root concavity. Incomplete root penetration appears in a radiograph as a dark continuous or intermittent linear shadow, the edges of which will usually be straight. Where welds are deposited without a root gap, lack of penetration may appear as a single continuous or intermittent dark line. It should be noted that root gaps frequently close during welding so even where there should have been a root gap lack of penetration may appear in the radiograph as a single dark line.

Figure 11.12 Lack of root penetration.

Non-metallic inclusions Usually formed by slag, but occasionally other foreign matter such as windblown sand may become entrapped within the molten weld material. Slag inclusions are irregularly shaped and may be either rounded/isolated or linear/elongated. Linear slag inclusions with a straight edge often indicate lack of fusion. Sometimes linear slag will appear on the radiograph as two parallel lines, often referred to as tram lines or wagon tracks. Most welding slag and other sources of non-metallic inclusions are radiographically much less absorbing than the surrounding metallic material; therefore they appear in the radiograph as dark images. Although very rarely used, some types of covered welding electrode have a high barium content in the flux coating and produce a slag radiographically denser than steel so slag inclusion may appear as a light image. Metallic inclusions Dependent upon the nature of the welding process it is possible for foreign metallic material to become entrapped within the molten weld material. Associated with the gas tungsten arc welding process, tungsten inclusions are probably the most commonly encountered form of metallic inclusion. They are caused by the break-up of the non-consumable tungsten electrode during welding and since tungsten has a melting point well in excess of 3000°C particles of tungsten falling into the weld pool do not become molten. Tungsten is radiographically extremely dense; therefore tungsten inclusions always appear as bright - light images which tend to be angular. They are usually quite small - typically around 0.5mm.


11-16


Copper inclusions can occur particularly with submerged arc or other welding process where the consumable electrode is fed through a copper contact. If the copper contact gets too near to touches the weld pool molten copper (melting point about 900°C) will become included in the weld pool. Copper is radiographically more absorbing than most other materials including steel so copper inclusions may produce light rounded images with extremely diffuse edges. Copper inclusions in ferritic steel welds usually cause severe transverse cracking. Metallic inclusions are quite common in aluminium welds, where such welds are not properly segregated from their steel counterparts. Aluminium melts at around 660C, steel above 1400oC so particles of steel or iron oxide falling into the weld pool will not become molten. Contamination can easily occur if tools such as grinding disks which have been used for steel are used on aluminium. Steel inclusions in aluminium appear as very bright angular shapes with sharp edges.

Figure 11.13 Slag inclusions.

Figure 11.14 Tungsten inclusion.

Gas pores: Porosity The solubility for gas of the molten weld material is many times that of the solid weld material, thus as the material freezes there is a tendency for any dissolved gases to precipitate from solution causing gas pores or porosity in the finished weld. Gas pores are extremely easy to detect by radiography since they are not sensitive to the direction of radiation and the gas which fills them is many times less radiographically dense than the surrounding material. Gas pores appear on a radiograph as sharply defined dark circular spots, and may be isolated, grouped or evenly distributed. Aligned porosity is usually an indication of lack of fusion. Evenly distributed porosity generally indicates that the electrode was faulty and group porosity usually occurs at restarts and is due to poor welding technique. Elongated cavities (hollow bead) These will generally only occur in the root run of welds deposited by manual metal arc welding. Welds deposited using cellulosic coated electrodes (AWS E6010, 7010, etc.) are more likely to suffer from this defect than welds deposited with other types of electrode.


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Hollow bead can be caused by holding the arc at too shallow an angle with respect to the work piece or by a strong draught of air along the inside of the pipe during welding. On the radiograph it looks very similar to slag - the radiographic indication usually has rounded ends and it is always situated along the centre of the root bead. Wormholes These are gas pores which have become frozen in the weld pool while attempting to migrate to the surface of the weld pool. In addition to occurring due to an excess of dissolved gas in the weld pool, they sometimes occur due to laminations in the parent material which extend to the weld face. Lack of fusion contains a small amount of entrapped air and this can cause wormholes in a similar way. Wormholes appear on the radiograph as a dark shadow, the shape of which depends on the orientation of the defect. If the wormhole is end on to the radiation a very dark rounded shadow is formed. It is side on then the appearance is somewhat like a tadpole. Where a lamination in the parent material or a lack of fusion is the source of wormholes they are often apparent in the radiograph in a herringbone-like array.

Figure 11.15 Wormholes due to a lamination in the parent material.

Crater pipes and cracks Occur due to shrinkage at the end of a weld run where the source of heat was removed too suddenly causing the weld pool to freeze too rapidly. It is quite common when the welding process is gas tungsten arc but it may also occur with shielded metal arc and other welding processes. A crater pipe will appear in the radiograph with an image very similar to that of a wormhole. It can only be distinguished from a wormhole by its position in the weld. Crater cracks are shrinkage cracks so have a relatively greater volume than most other cracks. They often have a star like appearance in a radiograph and their radiographic image rarely measures more than 3 or 4mm.


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11.8

Interpretation of casting radiographs Five groups of defect images may be seen in radiographs of metal castings:     

Voids. Cracks. Cold shuts. Segregation. Inclusions.

Voids Voids in castings are formed by gases dissolved in the molten material precipitating from solution during the solidification process or by shrinkage caused by inadequate feeding. Macroshrinkage (piping) Large cavity formed during the solidification process which occurs due to lack of sufficient feed material. With good mould design macro-shrinkage should be confined to the feeder heads. It appears on the radiograph as a dark continuous or semi-continuous area of varying film density with diffuse edges. Filamentary shrinkage (also called sponginess) Coarse form of shrinkage which has smaller physical dimensions than a macroshrinkage cavity. These cavities may be extensive and branching in nature and occur at the point in a casting which freezes last. Theoretically this should always be the centre of a section but this is not always the case, sometimes the defect may extend to the surface of the casting. Filamentary shrinkage has a diffuse branched appearance on the radiograph of variable film density. Microporosity/microshrinkage Very fine form of filamentary shrinkage due to lack of sufficient feed metal, gas or both, in which a number of cavities occur either round the grain boundaries or between the dendrite arms (a dendrite is a material crystal which in the initial stages of growth is tree-like). These cavities tend to link up in a three dimensional network throughout the material. In the radiograph the images of these cavities are superimposed and generally produce a mottled or cloudy effect. In non-ferrous alloys, particularly magnesium-based alloys, microshrinkage may occur in layers and produce dark streaks in the radiograph. Pinhole porosity Cavities less than 1.5mm diameter formed due to the evolution of gas from the molten material. The defect may be evenly distributed throughout the casting or localised in a particular area. When it is local to the surface of the casting, due to gas evolved at the mould face, it is known as subcutaneous pinhole porosity.


11-19


The defect appears in the radiograph as an assemblage of small, rounded, widely distributed dark images, distinguishable from microporosity by the size and rounded nature of the images which do not show the same tendency to interconnect. This defect can arise from the accidental injection of air during pressure die casting. Gas holes Discrete cavity greater than about 1.5mm diameter caused by gas evolved from the material as it freezes. It may also arise from gas evolved from the core or mould, in which case it is called blowhole. The radiographic image appears as a dark area of smooth outline which may be circular or elongated and can be associated with pinhole porosity. Gas holes occasionally become elongated as they try to rise to the surface of the molten material during cooling, in this form they are known as wormholes. The radiographic image of a wormhole may vary from a circular to an extremely elongated image depending upon the angle of view. Airlocks (entrapped air) Cavities formed by air trapped in the mould by the material during pouring. The defect appears in the radiograph as dark area with an outline which is generally smooth but which may have irregularities. An airlock cannot always be distinguished radiographically from a gas hole but a helpful guide to identification is the shape, size and position in the casting. In pressure die casting where air may be injected with the material the defect is usually more severe in the runners and may assume an angular form. In pressure and gravity die castings this defect may occur in clusters or as strings of small voids, whereas, in investment casting it may appear as small rounded voids. Cracks Cracks are discontinuities caused by fracture of the material at the point of solidification or some time thereafter. They appear on the radiograph as one or more dark lines. The width and form of the indication depends on the type of crack and radiographic technique used. Hot tears Discontinuities of a decidedly ragged form resulting from stress developed near the solidification temperature when a material has low mechanical strength. They usually arise when the natural contraction of the casting is restrained by the mould or core and occur mainly at or near a change of section. The defects are not necessarily continuous, they may exist in groups and will often terminate at the surface and are sometimes referred to as pulls. Radiographically hot tears are wavy, ragged dark lines, often discontinuous, with areas appearing as approximately parallel dark lines which may possibly be overlapping. Generally, the ends of the indication taper to become fine.


11-20


Stress cracks Well defined and approximately straight cracks formed after the material has become completely solid, quite large stresses being required to cause fracture. Distinctions are sometimes drawn between types depending on the time at which fracture occurred. In the radiograph stress cracks are often revealed as clearly defined smooth dark lines - thus differing from the ragged appearance of hot tears. Cold shuts Discontinuities caused by the failure of a stream of molten material to unite with either a confluent stream, or solid material, such as a chaplet, internal chill or pouring splash. In the radiograph these defects usually appear as dark lines and may be difficult to distinguish from hot tears except by the typical involute appearance of the end of the defects. The shape of an unfused chaplet or chill in a radiograph is dependent upon orientation of the beam. A cold shut resulting from a splash may appear as a dark crescent or circle. Inclusions Foreign matter (sand, slag, flux, dross, etc) entrapped in the casting. As an inclusion may be of greater or lesser opacity then the surrounding material it may appear radiographically as a light or dark area (eg a sand inclusion will appear dark in steel and light in aluminium). Slag usually gives a rounded image whereas material included in the casting as a solid (eg dross and sand), will give an irregular shape. If dross is trapped as an oxide film it will often produce a characteristic folded appearance in the radiograph inclusions may in many respects resemble voids in radiographic appearance but they will generally exhibit a greater variation in density. Segregations Result from local concentrations of any of the constituents of an alloy and may be classified as general, localised or banded. Detection of such defects by radiography depends upon the segregating constituents producing a local variation in the absorption of the radiation.


11-21


Section 12 Localisation

12

Localisation A radiograph is a two dimensional image of a three dimensional object. When a flaw is detected using a standard technique there is no certain way of telling how far below the surface the flaw is. In some cases it might be desirable to have three dimensional information about the position of a flaw. A technique called localisation can be used to estimate the through wall position of a volumetric flaw such as a slag inclusion. It is important to note that localisation of planar flaws such as cracks or lack of fusion is generally not possible by radiographic methods.

12.1

90 method The simplest method of localisation, but rather limited in its field of application. A typical test object, where this method might be useful would be a small to medium sized casting that has a fairly simple cross section. Figure 11.9 shows how this method would work on a small cylindrical object. Two radiographs are taken with primary beam mutually at 90 to each other. In an ideal situation the component would be placed on some kind of turntable so that it could be moved accurately keeping the two exposures in the same plane relative to the axis of the component. The apparent defect position in each radiograph can be measured relative to convenient datum point, the results plotted on a sketch and the defect position then deduced by triangulation.

12.2

Tube (source) shift method Figure 12.1 shows how this method could locate a slag inclusion in a butt weld. To work well a high degree of dimensional accuracy is needed. The source to object, the object to film distance and the distance that the source is moved between the successive exposures must all be accurately measured and controlled. Two half exposures are made from different source positions using the same radiographic film to produce two flaw images. The distance between the two images, is then measured and the flaw depth calculated as shown. The source shift, which is usually about one sixth of the source to film distance, has been exaggerated in the figure.

NDT20-71015 Localisation

12-1


Figure 12.1 The 90 method for a small cylindrical casting.


12-2


Figure 12.2 Tube shift method.

Note: Source shift distance exaggerated for clarity.


12-3


Using similar triangles: But so:

 A  d   x  s  B

m

 A  d   t  d  s  B

m

multiply both sides of the equation by Bm: Am + dm = tB - dB + sB add dB and subtract Am both sides: dB + dm = tB + sB - Am which can be written as: d(B + m) = B(t + s) - Am divide both sides by (B + m):

d 12.3

B t  s   Am B  m 

Tube (source) shift method with lead markers Placing lead markers on the component source and film side as shown in Figure 12.3 removes the need for accurate measurement of the source to object, object to film distances and the distance that the source is moved between the successive exposures. Refer to Figure 12.3 below. The three triangles in the enlarged view will be very similar as long as the source or focus to film distance is long in relation to with the thickness. If the triangles are similar then:

a b c   x y z So it follows that: a 

CX cy and : b  Z z

We also know that: c - a = t and: c - b = d Therefore we can write: c  t 

CX cy and : c  d  Z z

(Argument continued below Figure 12.3).


12-4


Figure 12.3 Tube shift method with lead markers.


12-5


We have already established that: c  t 

From this we can see that: d  C 

And: t  c 

CX cy and c  d  Z Z

cy Z

CX Z

 

y  Z

(1)

 

X  Z

(2)

So: d  c1 

And: t  c 1 

Now divide Equation (1) by Equation (2) to get:

y  1   d  Z  X t  1   Z  t, the thickness of the plate is known and x, y and z can be measured on the radiograph. Therefore d can be calculated.


12-6


Section 13 Units Used in Radiography

13

Units Used in Radiography

13.1

Ionisation (exposure) The quantity of ionising radiation can be measured in terms of its ionising effect or exposure on air at standard temperature and pressure (STP). The SI unit of exposure is the coulomb per kilogram in air, the quantity of ionising radiation that produces a total electric charge of 1 coulomb per kilogram (Ckg-1) of air at STP. The centimetre-gram-second (CGS) unit of ionising effect is the roentgen (R), the quantity of ionising radiation that produces an electric charge of 1 electrostatic unit (ESU), equivalent to 2.08 x 109 ion pairs, per cubic centimetre of air at STP. One cubic centimetre of air at STP weighs 0.001293g. One ESU is equal to 3.336 x 10-10 coulomb so: 1R = 2.58 x 10-4 Ckg-1 (in air) or 1 Ckg-1 = 3876 R Note: Unit of exposure is defined for air, hence cannot be used directly to describe the dose to tissue.

13.2

Absorbed dose Is defined as mean energy imparted to matter by ionising radiation per unit mass of irradiated material. The SI unit of absorbed dose is the gray (Gy). One gray represents absorption of 1 joule of energy per kg of irradiated material. The CGS unit of absorbed dose is the roentgen absorbed dose (rad). One rad represents absorption of 100 ergs of energy per gram of irradiated material. 1 Gy = 100 rad The units of radiation absorbed dose can be approximately related to the units of ionising effect. One Roentgen corresponds to 1.61 x 1015 ion per one kg of air which has than absorbed 8.8 mJ. 1R = 0.88 rad 1 Ckg-1 = 3411 rad = 34.11Gy The conversions above are approximate since the relationship between the roentgen and the rad or the coulomb per kilogram and the gray varies to some extent with radiation energy. Note: Absorbed dose doesn’t describe the biological effect to different radiations

NDT20-71015 Units Used in Radiography

13-1


13.3

Man mammal equivalent or radiobiological equivalent The effect which ionising radiation has on our bodies varies with the type of radiation and also, to some extent, with radiation energy. Also the chemical and biological effectiveness of a particular radiation depends on the distribution of the absorbed energy within the medium. In order to compensate for this a quality factor (QF) is introduced. Quality factors for several types of ionising radiation are listed in Table 12.1 below. Table 12.1 Quality factors. Type of radiation Quality factor, QF X-rays 1.0 Gamma rays 1.0 Beta particles 1.0* Alpha particles 20 Thermal neutrons** 2 Fast neutrons*** 10 Protons 10 Heavy ions 20 * may in some cases exceed 1.0 ** energy < 10keV *** energy > 10keV

In order to compare the biological effectiveness and also for radiation protection measure the quantity “dose equivalent” is used. The SI unit of dose equivalent is Sievert (Sv) and is given by: Dose equivalent (Sv) = Absorbed dose (Gy) x quality factor (QF). In CGS system dose equivalent is given in terms of rem, defined by Dose equivalent (rem) = Absorbed dose (rad) x quality factor (QF). Also 1Sv = 100 rem In the CGS system multiplying the dose in rad by the appropriate quality factor gives the dose in roentgen equivalent man (Rem) where 1Rem is the amount of ionising radiation which has the same biological effect as 1rad of X-rays. In the SI system multiplying the dose in gray by the appropriate quality factor gives the dose in sievert (Sv) where 1Sv is the amount of ionising radiation which has the same biological effect as 1Gy of X-rays. Thus: 1Sv = 100Rem or 1Rem = 0.01Sv


13-2


13.4

Dose rate Dose rate in the SI system is generally measured in microsieverts per hour (μSv/h), but may also be measured in millisieverts (mSv) or sieverts (Sv) per hour. Alternatively dose rate can be expressed in micrograys (mGy), milligrays (mGy) or grays per hour. In the cgs system dose rate is generally measured in millirem per hour (mRem/h) but may be measured in Rem per hour (Rem/h). 1 mRem/h = 10 μSv/h or 1 mSv/h = 0.1 mRem

13.5

Source strength or activity For radioactive sources the source strength or activity is the number of disintegrations occurring each second and is proportional to the number of active atoms present in the source. The cgs unit of source strength or activity is the curie (Ci). One curie is equal to 3.7 x 1010 disintegrations per second. The SI unit of source strength or activity is the becquerel (Bq) or the gigabecquerel (GBq). One becquerel is equal to one disintegration per second; one gigabecquerel is equal to 109 disintegrations per second. 1Ci = 37GBq or 1GBq = 0.027Ci

13.6

Specific activity The specific activity of a radioactive source is equal to the source activity divided by the weight of the source. In the cgs system it is expressed in curies per gram (Ci/g) while in the SI system it is expressed in becquerels per gram (Bq/g) or gigabecquerels per gram (GBq/g).

13.7

Output The output of a source of ionising radiation is the dose rate per hour at some fixed distance, usually 1m from the source. For radioactive isotopes it is useful to state output in grays, sieverts, rads or Rems per hour per curie at 1m. Table 12.2 gives some examples. Table 12.2 Output of various radioactive isotopes. Isotope name Thulium 170 Ytterbium 169 Selenium 75 Iridium 192 Cobalt 60


Output, mSv per hour per Ci 0.026 1.25 1.8 4.8 13.0

13-3


The output of radiation from a typical 200kV industrial constant potential X-ray machine is as much as 1,000mSv per milliampere of tube current at a distance of 1m from the focal spot.


13-4


Section 14 Radiation Monitoring Devices

14

Radiation Monitoring Devices Ionising radiation cannot be detected by humans and is extremely harmful to health therefore it is imperative we have reliable equipment that can measure the radiation dose. Two basic types of radiation monitoring device exist: (1) Those which give a read out of the current dose rate and (2) devices which measure the accumulated dose over a given period of time.

14.1

Survey meters Survey meters give a real time measurement of dose rate. There are five basic types, ionisation chambers, proportional counters, Geiger counters, scintillation counters and solid state devices. Each of these is discussed and described in the sections below.

14.1.1 Ionisation chambers An ionisation chamber is part of the family of radiation detectors known as gaseous detectors. The ionisation chamber can take many forms, but basically consists of two electrodes separated by a layer of gas. As ionising radiation interacts with the gas, causing ionisation, it becomes electrically conductive and pulses of current flow as each photon of ionising radiation is received. Compared with other types of gaseous detector the ionisation chamber operates at low electrical voltage, see Figure 14.1. The actual voltage needed depends on the geometry and size of the ionisation chamber. Ionisation chambers can detect alpha, beta and gamma or x-ray radiation but give no information as to the photon energy of the radiation detected. They are occasionally used in conjunction with an electronic circuit that counts the current pulses but usually the output is a reading of the average current flowing across the chamber. The measurement range of ionisation chamber instruments is comparatively narrow and they tend to be bulky and fragile compared with the Geiger counter so are seldom seen in industrial applications.

Figure 14.1 Gaseous detectors, pulse size versus applied voltage.

NDT20-71015 Radiation Monitoring Devices

14-1


Figure 14.2 Simplified layout of an ionisation chamber.

14.1.2 Proportional counters Neither the Geiger counter, nor the ionisation chamber can give any information as to the photon energy of the ionising radiation received. The best that can be achieved with them is to shield the chamber such that alpha and beta radiation are excluded from the measurement. The gas chamber used in a proportional counter often contains multiple electrodes. Proportional counters operate in a voltage range between the ionisation chamber and the Geiger counter. In addition to gauging radiation dose rate or intensity they are able to give information as to the type and photon energy of the radiation received and are also able to determine the direction from which the radiation is coming. They are often used as fixed monitoring instruments within and around nuclear installations, but are rarely seen in other workaday industrial applications. 14.1.3 Geiger counters Geiger counters operate at higher voltages than the proportional counter; typical operating voltages vary from 400-1000V or more dependent on the size and geometry of the gas chamber. At such voltages the pulse size is very large and no amplification is needed. The original 1928 version of the Geiger tube contained a special self-quenching gas mixture consisting of an inert gas doped with a small amount of hydrocarbon (eg butane). This was greatly improved in 1947 when Liebson designed a tube containing inert gas with a small proportion of halogen (eg bromine). All modern instruments follow the Liebson design. Geiger tubes can be made very small; a cylinder of less than 6mm diameter and length 25mm is not untypical and are extremely durable and reliable. A Geiger tube constructed of a light metal such as aluminium will detect only x- or gamma rays. Tubes provided with a window made from thin glass will also detect beta radiation while those having a window made from mica can detect alpha in addition to beta and gamma. The measurement range of the instrument can be extended by shielding the tube. Geiger tube instruments are otherwise insensitive to changes in photon energy. In general Geiger counters give little information as to the direction from which the detected radiation is coming. They may give a reading in counts per second, but usually the average current flowing across the tube is measured with the ammeter scale being calibrated to read microsieverts or millisieverts per hour. As radiation intensity increases to high levels a Geiger counter will become increasingly inaccurate, because it suffers from a short dead time after a pulsing event has occurred – if another photon of radiation arrives during the dead time it will not be detected. Some instruments will cease to function if exposed to a very high dose rate.


14-2


14.1.4 Solid state radiation detectors Solid state radiation detectors have been available since the 1950s. Various types of semiconductor are available which begin to conduct electricity under the influence of ionising radiation. Instruments based on this type of semiconductor are able to differentiate between different photon energies. Thus in addition to measuring dose rate they can provide information as to the spectrum of radiation that is present. 14.1.5 Scintillation counters Various materials known as phosphors emit flashes of light when placed in a beam of ionising radiation. Phosphors can be manufactured to respond to one or more types of ionising radiation. Table 14.1 lists some common phosphorescent materials, many others exist, including a number of organic liquids and solids. Phosphors have been used as radiation detectors since the very early days of the discovery of ionising radiation; both Roentgen and Becquerel used them. The amount of light produced can be quite small so phosphors are always used in conjunction with a light amplification system such as the photomultiplier tube. Modern instruments use charge coupled devices (CCDs) in conjunction with a radiation sensitive phosphor. A CCD is at the heart of any modern digital camera. Those used for radiation detection measure the intensity of light emitted from the phosphorescent layer under the influence of ionising radiation. Whichever system is used, scintillation counters relate the intensity of light produced by the phosphor to the intensity of the ionising radiation received. Generally they give a reading in counts per second but occasionally they will be calibrated to read directly in microsieverts or millisieverts per hour. Scintillation counters are extremely sensitive, and can detect very low levels of ionising radiation. They are direction sensitive and very useful when searching for radioactive contamination. They are used in industrial radiography to check for leakage of fissile material from a sealed source. Table 14.1 Common phosphorescent materials. Phosphor (activator) Sodium iodide (thallium) Lithium iodide (europium) Zinc sulphide (silver) Bismuth germanate (N/A)

14.2

Sensitive to: Gamma Gamma and neutrons Alpha Gamma

Personal monitors Survey meters, with a few exceptions, give a real time reading of dose rate but do not integrate this to give a total dose received over a given period of time. Several types of device exist which integrate the dose received over a period of time. One convenient use of such a device is for monitoring the total dose that a person receives during the course of his working day. When used in this way such devices are referred to as personal monitors. Four types of personal monitor are commonly used in industrial radiography.


14-3


14.2.1 Film badges Monitoring film Plastic filter

Copper filters

Open window Aluminium filters

Plastic filter Figure 14.3 Film badge.

The principle of a film badge is that when exposed to ionising radiation followed by developing under tightly controlled conditions the film density produced can be related to the radiation dose received. Film badges, as shown in Figure 14.3, can be used to detect x-, gamma and beta radiation. Coupled with the right type of intensification screen radiographic film can be used to detect and measure other types of ionising radiation. The film badge of the type shown in Figure 14.3 contains a section of carefully manufactured radiographic film having two emulsions, one fast and one slow. The use of two emulsions extends the measurement range of the badge. The badge holder is equipped with various filters which extend the range of measurement and additionally enable the badge to give some information as to the type and photon energy of the ionising radiation received. The film badge has, in large part, been replaced by the thermoluminescent dosimeter (TLD) (see below). Table 14.2 gives a comparison of typical film badge and TLD specifications. Table 14.2 Film badge and thermoluminescent dosimeter specifications. Film badges Radiation type Gamma Measuring range 10keV to 7MeV (photon energy) Measuring range 0.1mSv to 10Sv (dose) Typical period of 2 to 4 weeks use Thermoluminescent dosimeters Radiation type Gamma Measuring range 10keV to 10MeV (photon energy) Measuring range 0.05mSv to 10Sv (dose) Typical period of 4 weeks use


14-4

X-ray 10keV to 7MeV

Beta 700keV to 3.5MeV

0.1mSv to 400mSv

0.1mSv to 10Sv

X-ray 10keV to 10MeV

Beta 700keV to 3.5MeV

0.05mSv to 10Sv

0.05mSv to 10Sv


14.2.2 Thermoluminescent dosimeters (TLD) Thermoluminescent dosimeters offer several significant advantages over the film badge:     

Much less easily damaged. Slightly wider measurement range. Much less subject to possible errors or failures in processing – the measurements obtained have a better degree of accuracy. Can be reused many times. Absorption characteristics of the TLD more closely resemble those of the human body, thus dose calculations are simplified.

Most TLD badges contain two or more discs of a thermoluminescent material, usually lithium fluoride but occasionally other materials are used. During exposure to ionising radiation lithium fluoride stores energy, when subsequently heated to a temperature of around 250C the stored energy is released as flashes of light. The number of flashes can be counted and this is directly related to the radiation dose received. TLD badges are worn in specially designed plastic holders similar to those used for film badges. The addition of plastic or aluminium filters extends the measurement range and facilitates the obtaining of information concerning the photon energy and type of radiation. 14.2.3 The quartz fibre electrometer (personal dosimeter) These devices are still widely used in the USA where in many states they are mandatory wear for all personnel involved in working with ionising radiation. In the UK they used to be popular for use inside nuclear power plants but they have now largely been replaced by more reliable and accurate solid state devices.

Figure 14.4 Quartz fibre electrometer.

The quartz fibre electrometer (QFE) is a gaseous detector like the ionisation chamber, proportional counter and Geiger counter described above. When raised to the light a scale like on the right of Figure 14.4 can be seen through the lens of the instrument. The vertical line is the quartz fibre. When a static electrical charge is applied to the instrument the quartz fibre moves to the zero point of the scale. As the gas inside the QFE becomes ionised the static charge is gradually dissipated and the fibre begins to move to the right. The corresponding total dose received can be read on the upper scale.


14-5


The QFE has quite a narrow measuring range, typically 0-50mSv or less. The example shown above has a measurement range of 0-200mRem, equivalent to 0-2mSv. The QFE is sensitive to x- and gamma radiation in the photon energy range 45keV to 3.5MeV and is a very convenient means for checking how radiation doses are accumulating during a working day but it suffers from fragility and is very easily damaged. 14.2.4 Solid state integrating dosimeters The QFE has largely been replaced by solid state integrating dosimeters which are extremely shock-proof and have a wider measuring range. They are typically combined with an audible warning device which bleeps if the wearer enters a high radiation area.


14-6


Section 15 Radiation Safety

15

Radiation Safety All personnel working with ionising radiation should be aware that it is injurious to the human body, or any other biological tissue. Anyone working where radiography is carried out should make themselves fully aware of the safety procedures and regulations in force and take care to observe all warning barriers. Ionising radiation cannot be detected by the five human senses and has cumulative effects upon the human metabolism and causes genetic damage to the human body, the full effects of which may not be apparent until 15-35 years after the initial exposure. Regardless of any nominal safe limits it is always prudent to avoid exposure to radiation whenever possible. NB Where industrial radiography is concerned there is little or no danger from contamination because all gamma sources in use are of the sealed variety. x- or gamma-rays are not capable of producing any residual radioactivity in the items subjected to exposure.

15.1

Precautions

15.1.1 Exposure booths At locations where a large volume of industrial radiography is carried out exposure booths of various shapes and sizes will generally be available, which usually consist of enclosures having lead lined walls. Some exposure booths have walls filled with spent casting sand or other radiation absorbing material. Such exposure booths should be regularly monitored to ensure that the radiation dose rate is within safe limits in the areas outside the booth where personnel can move freely. Safety switches are usually fitted to doors of exposure booths to prevent the operation of x-ray sets or gamma ray equipment whilst the door is open. Where overhead cranes might have to pass over an open topped exposure booth similar safety switches are normally installed o trip out the x-ray set, or wind back the gamma ray source, should the crane encroach upon the irradiated area during exposure. In many countries, including Britain, it is a legal requirement that an audible warning is given before any exposure takes place. Exposure booths should be equipped with switches inside the x-ray compound which can be operated to prevent the operation of the x- or gamma ray equipment should any personnel be accidentally trapped inside. Radiation detectors should be installed inside the exposure booth to indicate when gamma ray sources are being used. 15.1.2 Site work A demarcation barrier is required showing the zone where radiation is in excess of the legally permitted limit (in Britain this is 7.5μSv/h). The barrier usually consists of brightly coloured rope or tape suspended at about 1m above the ground, with warning signs at 5m intervals. Areas which will be irradiated at greater than the legal limit must be cleared of all non-classified personnel prior to any exposure. Audible and visible warnings must be given before any exposure takes place. The barrier should be monitored with an efficient radiation detector and should be guarded by classified personnel during exposure. NDT20-71015 Radiation Safety

15-1


15.1.3 Scatter Personnel should be aware that radiation can be scattered by structures, apparently safe locations may be subject to stray scattered radiation. 15.2

Exposure limits for radiation workers In Britain classified workers are allowed to receive an accumulated dose of 20 millisieverts (20mSv) per year from the age of 18 to 65 years. A formal investigation is required if a classified worker receives a dose of 15mSv or more within any single calendar year. The investigation has to establish the source of the dose and may include a thorough medical check for the person receiving the dose. These requirements are typical for all countries within the European Union, but requirements in other countries may differ widely.

15.2.1 Dosimeters For work in radioactive environments (ie nuclear reactors) personnel must be equipped with direct reading dosimeters which will display immediately the accumulated dose received. Personnel working in these locations must take particular care to avoid ingesting radioactive particles. Tightly fitting breathing masks are required and protective clothing should be worn. 15.3

Permitted levels The figures given below relate to Statutory Instrument 1999 Number 3232, Ionising Radiation Regulations 1999. These regulations exclude radiation doses received due to medical reasons.

15.3.1 Classified workers The maximum permitted dose rate for personnel equipped with film badges (or TLDs) is 20mSv per year, approximately equivalent to a constant dose rate of 10μSv/hr for a 40 hour working week if a 48 working week year is assumed. 15.3.2 Unclassified personnel, controlled and supervised areas Controlled area Unclassified personnel must be excluded from any area where radiation dose is deemed likely to exceed three tenths of the annual allowable dose for a classified worker (6mSv). The maximum permissible dose rate at the boundary of a controlled area is 7.5μSv/hr. Supervised area Defined as an area where the annual dose is expected to equal or exceed 1mSv. Such areas should be clearly signed; unclassified persons are permitted to pass through such areas but must not remain in them for extended periods. Verbal warnings should be given by the radiographer where possible. 15.4

Safe working distances The dose rate from a source of ionising radiation reduces in proportion to the reciprocal of the square of the distance from the source.

NDT20-71015 Radiation Safety

15-2


For any source of ionising radiation: Dose rate =

If the source of ionising radiation is x-ray then it will not be possible to calculate the dose rate at 1m although the dose rate will be proportional to the tube current. Halving the tube current at a given tube voltage will halve the radiation dose rate. If the source is gamma ray then the dose rate at 1m can be calculated if the source strength (curies or gigabecquerels) and output of the source are known. Output for any given isotope is the dose rate per curie or gigabecquerel at 1m from the source. Thus: Dose rate at one metre - (source strength) x (output) Output for the various radioactive isotopes used in industrial radiography is tabulated below: If we take 7.5μSv/hr to be the safe dose rate then we can calculate the safe distance using the formula below: Safe distance in metres =

(Output )  Source strength   1,000  7 .5

The previous formula can be simplified to: Safe distance in metres x C x Source strength in curies C is a constant for each isotope, for thulium 170 C = 1.86, ytterbium 169 C = 12.91, selenium 75 C = 15.49, iridium 192 C = 25.30 and for cobalt 60 C = 41.63. 15.4.1 Shielding If shielding is introduced then the reduction in the minimum safe working distance can be calculated if the magnitude of the half or tenth value layer of the shielding material is known. The half value layer for any material is the thickness of material that will reduce the radiation dose rate, for a given radiation energy, by a factor of two. The tenth value layer is similarly the thickness of material that will reduce the dose rate by a factor of ten. For example the half value layer of lead for cobalt 60 is about 12.5mm while for iridium 192 it is about 4.8mm. The tenth value layer of lead for cobalt 60 is about 41.5mm while for iridium 192 it is about 16mm. If the shielding thickness is an exact multiple of the half or tenth value layer then the dose rate after shielding can be found simply by dividing the unshielded rate by two for each half value layer or by ten for each tenth value layer. Where this is not the case the formulae given below can be used.


15-3


RS 

RU

2

t / hvl 

or RS 

RU 10 t / tvl 

Where: Ru = Unshielded dose rate. Rs = Shielded dose rate. t = Thickness of shielding material. hvl = Half value layer. tvl =Tenth value layer.


15-4


Glossary

Glossary The following is a compilation of the more common terms used in connection with radiographic testing. Other lists can be found in BS EN 1330-3 and ASME V, Appendix A. Absorbed dose

(of ionising radiation) Energy per unit mass imparted to the irradiated material. Absorbed dose is measured in Grays (Gy) (1 Gy = J kg-1).

Absorption

Reduction in intensity of a beam of radiation during its passage through matter.

Absorption coefficient

Usually abbreviated μ. Where I = I0 x e-μt and I is the shielded radiation intensity, I0 is the unshielded radiation intensity while t is the thickness of the absorber.

Alpha radiation

Type of ionising radiation consisting of high velocity charged particles emitted from the nucleus of heavy radioactive isotopes. The alpha particle consists of two protons and two electrons – a helium nucleus, and has a positive charge of 2. Alpha radiation has very low penetrating power, but is very strongly ionising.

Anode

Positive electrode of a discharge tube. In an x-ray tube the anode carries the target (see target).

Atom

Smallest indivisible part of a chemical element consists of a nucleus formed from positively charged protons and neutrons surrounded by orbiting negatively charged electrons.

Atomic mass number

Total number of protons plus neutrons in the nucleus of an atom, standard abbreviation A.

Atomic mass unit (AMU)

Measure of atomic weight, roughly speaking the proton has a weight of 1amu while that of the neutron is marginally greater than 1amu. The weight of the electron is roughly 0.00054amu.

Atomic number

Number of protons in an atomic nucleus, standard abbreviation Z.

Atomic weight

Weight of an atom expressed in atomic mass units, approximately equal to the atomic mass number.

Backscatter

Scattered radiation caused by the presence of objects behind the film or radiation detector (see scatter).

Background count

Ionising radiation dose rate due to natural causes (see background radiation).

NDT20-71015 Glossary

1


Background radiation

Ionising radiation present at any given site due to natural causes: sunlight contains a proportion of ionising radiation; some natural rocks such as granite are weakly radioactive.

Base fog level

Film density of a radiographic film prior to exposure to ionising radiation (measured by processing a sample of unexposed film). Base fog level increases with the age of the film. Poor storage conditions, temperature or humidity too high, are common causes of excessive base fog level. Other possible causes of high base fog level are exposure to chemical or solvent fumes, ionising radiation or to light. Most international standards specify a maximum base fog level of 0.3.

Becquerel

SI unit of radioactivity which is defined as 1 disintegration per second (1 curie = 37GBq (gigabecquerels)) (see curie).

Beta radiation

Ionising radiation consisting of very high velocity electrons emitted from the nucleus of a radioactive isotope. In beta emission a neutron converts to a proton while emitting a very high speed electron. Beta radiation has low penetrating power but must not be ignored when assessing radiation safety as it causes severe skin burns and can lead to fatality.

Betatron

Apparatus in which electrons are accelerated along a spiral path by means of the electric force associated with a varying magnetic field.

Bunsen-Roscoe law

Reciprocity law which basically states that the film density produced by a dose of ionising radiation is independent of the radiation dose rate (ie a low dose rate for a long exposure time will produce the same film density as a high dose rate for a short exposure time so long as (dose rate) x (time) remains constant).

Build-up

As ionising radiation is scattered the majority of the scattered radiation continues in approximately the same direction as the primary beam. When working with radiation shielding thicknesses amounting to several half value layers this causes the transmitted radiation intensity to be higher than would be expected based on the number of half value layers. The problem of build-up increases as radiation energy increases.

Calcium tungstate

Complex salt of calcium which fluoresces in the blue part of the visible light spectrum during exposure to ionising radiation. Calcium tungstate is the common base material for salt or fluorometallic intensifying screens.


2


Cassette

Lightproof container for holding x-ray film during exposure may be rigid or flexible. They must be designed to maintain good contact between the xray film and intensifying screens.

Casting

Formation, by pouring molten base material into a mould, of a useful product shape, or any component produced by such a process. The great majority of metallic components begin life as a casting.

Cathode

Negative electrode of a discharge tube which usually consists of a heated tungsten filament.

Characteristic curve

The curve for a given photographic (or x-ray) film which relates the logarithm to the base of 10 of the relative amount of radiation exposure (ie radiation intensity x exposure time) to the achieved photographic density under specified processing conditions. Also known as Hunter & Driffield curve (H&D curve) or sensiometric curve. The process by which a characteristic curve is produced is called sensiometry.

Compton scattering

Important scattering mechanism in industrial radiography which is predominant for x-ray photon energies between 0.6-6MeV.

Constant potential

Uni-directional voltage constant) magnitude.

Contrast

Difference in brightness to the human eye of two adjacent areas in a radiographic image.

Curie

Unit of radioactivity for any radioactive isotope. A radioactive isotope with source strength of 1 Curie is decaying at the rate of 3.7 x 1010 disintegrations per second. (Abbreviation: Ci). The SI unit of radioactivity is the Becquerel (see Becquerel).

Curie-hours or curie-minutes

Gamma ray exposures are usually expressed in curie hours or curie minutes because the intensity of radiation is proportional to the source strength and the total amount of radiation received is proportional to the source strength multiplied by the exposure time.

Definition

Degree of sharpness of delineation of image detail in a radiograph (see geometric unsharpness, inherent unsharpness and penumbra).

Densitometer

Instrument used for measuring radiographic density (see density).


3

of

constant

(or

nearly


Density

Degree of darkness of a radiograph expressed as the logarithm to the base 10 of the ratio of the intensity of incident light to intensity of the light transmitted through the film.

Density strip

Strip of film exposed to form gradations of film density, once calibrated using a suitable densitometer they form a convenient means of comparing film densities. Alternatively density strips of known density can be used to check the calibration of densitometers.

Developer

Chemical solution used in the development of a radiographic film. All developers are reducing agents which reduce the sensitised silver halide grains in the film emulsion to metallic silver thereby producing an image on the film.

Development

Chemical process by which a latent image is converted to form a visible image (see developer).

Die casting

Casting process for producing small to medium sized components, mainly applicable to low melting point alloys, involving the use of a reusable mould generally constructed from steel. In pressure die casting the molten charge is forced into the mould under pressure. Die castings have a better surface finish and mechanical properties than equivalent sand castings.

Dose rate

Total quantity of radiation energy per unit time. It usually is expressed in Sieverts or Rems per unit time (see sievert and Rem).

Electromagnetic radiation

Light waves, radio waves and ionising radiation (xor gamma rays) are all forms of electromagnetic radiation. All electromagnetic radiation travels at the same velocity (299,274,000m/sec in a vacuum), the different types of electromagnetic radiation differ only in their wavelengths.

Electron

Very tiny negatively charged fundamental particle. The mass of the electron is roughly 0.00054 times that of a proton.

Electron volt

Unit of energy equal to the amount kinetic energy acquired by an electron when it is accelerated through a potential difference of one volt. (Abbreviated as eV 1keV = 1,000eV, 1MeV = 1,000,000eV). Electron volts are a convenient unit of expressing ionising radiation energies.

Emulsion

Photographic emulsion; a suspension of photosensitive material (silver halide grains) in a matrix such as gelatine.


4


Exposure chart

Chart which relates the required radiographic exposure for a given radiographic film and film density to the penetrated thickness of a specified material.

Filament

In an x-ray tube the heated cathode usually consists of a thin tungsten wire through which a heating current is passed in order to stimulate the thermionic emission of electrons.

Film badge

Piece of photographic film used as a radiation monitor. Film badges are usually partially shielded to increase the effective measuring range.

Film contrast

The degree to which a particular radiographic film when viewed by the human eye can differentiate between two adjacent areas of different radiation exposure. Film contrast is related to the change in film density per unit increase in radiation exposure and increases with the achieved film density.

Filter

Material, usually thin copper sheet, interposed in the path of radiation in order to reduce selectively the intensity of radiation of a certain range of wavelengths or energies (usually the lower range of energies). Filters are useful for reducing the effect of scattered radiation in x-radiography.

Fixer

Chemical solution containing principally sodium or ammonium thiosulphate which takes into solution the excess silver halides in a film emulsion which remain after the development process has been completed.

Fixing

Chemical removal of unused silver halides from an emulsion after development (see fixer).

Flaw sensitivity

Ability of a radiographic technique to detect flaws. Not easy to quantify but is expressed as the minimum detectable thickness of a specific flaw measured in the direction of the radiation beam, expressed as a percentage of the total thickness of a specimen of specified homogeneous material.

Fluorescence

Ability of certain chemical compounds to convert invisible incident radiation to a visible radiation emission. Calcium tungstate is useful in radiography because it fluoresces in the blue part of the light spectrum under the action of ionising radiation.

Fluorescent screen

Suitably mounted layer of material (eg calcium tungstate, barium platino-cyanide or zinc sulphide) which fluoresces in the visible region of the spectrum under the action of ionising radiation: Fluorescent screens can be used to produce a radiographic image directly or to produce an intensifying effect when used in conjunction with radiographic film.


5


Fluorometallic screen

Intensifying screen used in radiography which combines the scatter reducing properties of a lead screen with the image intensification properties of a salt screen.

Focal spot

Area of the target on which the electron stream impinges and from which x-rays are emitted. The effective focal spot size in an x-ray tube is usually less than the actual focal spot size.

Focus-to-film distance

Distance from the focus of an x-ray tube to a film set up for radiographic exposure, abbreviation: FFD.

Gamma radiation

Electromagnetic radiation emitted during the decay of some radioactive isotopes. Gamma photons are the by-product of some alpha or beta decay events. All gamma emitters used in industrial radiography emit gamma as a by-product of beta decay.

Gamma radiography

Radiography by means of gamma rays.

Gray

SI unit of absorbed dose.

Half life

Fixed property of any radioactive isotope. Half life is the time taken for the number of radioactive atoms in a given sample to reduce by half. It can be anything from a few seconds to millions of years dependent on the isotope. The decay process itself is random but the half life is fixed because each radioactive atom has the same probability of decay – averaged over a very large number of radioactive atoms this gives rise to a fixed half life.

Half-value thickness (half-value layer)

Thickness of a specified substance which when introduced into the path of a given beam of radiation reduces the radiation intensity by half. It may be used as an indication of the quality of the beam or the opacity of the substance, abbreviation: HVL.

Identification marker

Marker, usually of heavy material (lead), used to provide a reference point or identification mark in a radiograph.

Image intensifier

Device used in fluoroscopy (form of real time radiography), incorporating a vacuum tube in which electrons released by x-rays from a special type of screen, are accelerated and focused on to a fluorescent screen thus producing a brighter image than that produced directly using a fluorescent screen.


6


Image quality Indicator

Any device which gives an indication of radiographic quality. The commonest are wire, step hole and plaque IQIs. The quality (or sensitivity) of the image is defined as the smallest discernible wire diameter or step thickness expressed as a percentage of the total thickness.

Inherent fog

Unwanted blackening of an emulsion caused by the development of grains which are inherently developable without exposure. This type of fog varies with the age of the emulsion and conditions of storage (see base fog level).

Intensifying factors

Ratio of exposure time without intensifying screens to that when screens are used, all other conditions being the same.

Intensifying screen

Layer of suitable material, eg lead foil, which when placed in close contact with photographic emulsion, adds to the photographic effect of the incident radiation.

Injection moulding

Die casting process whereby molten raw material is forced into a mould under pressure. The process is common for plastics and low melting point metal alloys, especially those of zinc, magnesium and aluminium.

Investment casting

Also called the lost wax process. A wax model of the required item is made and a mould is formed around this using some type of refractory material. After firing to harden the mould and burn out the wax, molten metal is poured in to produce the desired component. Investment casting is an expensive method but it produces the best surface finish and material properties of all of the casting processes.

Isotopes

Nuclides having the same atomic number but different mass number (ie the nucleus contains the same number of protons but a different number of neutrons). Some isotopes are stable while others undergo nuclear fission thus producing emissions of radiation.

Latitude

Ability of a radiographic technique to display a wide range of material thickness at an acceptable film density. In general latitude will be reduced if a high contrast ultrafine grain film is used.

Macroradiography

Radiography of thin sections of material in such a way that the resulting image may be enlarged to reveal microstructure.


7


MAG

Metal active gas welding, often referred to as CO2 welding, mainly applicable to carbon steel. MAG is an automatic or semi-automatic arc welding process involving a reel fed consumable electrode; the arc is shielded by an active gas, usually CO2. Porosity and lack of sidewall fusion are common defects.

MIG

Metal inert gas welding. MIG is an automatic or semi-automatic arc welding process involving a reel fed consumable electrode; the arc is shielded by an inert gas, usually argon. MIG welding is applicable to most metals and alloys. Porosity and lack of sidewall fusion are common defects.

Milliampere hours, minutes or seconds

Measure of x-ray exposure expressed as the product of the milliammeter reading (ie tube current) and the of exposure time in hours, minutes or seconds.

MMA

Manual metallic arc welding. An arc is struck between a flux coated consumable electrode and the work piece, the flux coating decomposes to form a shielding gas, usually carbon dioxide and a molten slag which protects the hot metal.

Neutron

Fundamental particle having a mass slightly greater than 1amu and zero electrical charge.

Neutron radiography

Neutrons emitted by nuclear reactors and some radioactive isotopes are a form of penetrating radiation and can be used to perform radiography. Neutrons are heavily absorbed by substances such as water or plastic which contain significant amounts of hydrogen and pass easily through metals such as steel or aluminium. Neutron radiography is useful for the detection of water ingress into aeroplane wing structures and other similar applications.

Pair production

The scattering mechanism which predominates at xray photon energies exceeding 6MeV. Pair production does not occur until the threshold photon energy of 1.02MeV is exceeded.

Penumbra

Partial shadow extending beyond the edges of the main shadow (umbra) of an object due to the finite size of the radiation source: the width of this partial shadow.

Photoelectric effect

Important scattering mechanism in industrial radiography which is predominantly a radiation using photon energies of below 0.6MeV.

Positron

Basically an electron but with opposite electrical charge, they are emitted during pair production which is an important scattering mechanism in high energy x-radiography.


8


Pressure mark

Variation in photographic density caused by the application of local pressure to the emulsion; the mark may be light or dark according to circumstances.

Primary radiation

Radiation which is incident on the absorber and which continues unaltered in photon energy and in direction after passing through the absorber.

Processing

Series of operations, such as developing, fixing and washing, associated with the conversion of a latent image into a stable, visible image.

Quality factor

To account for the fact that, for instance 1Gy or 100R of alpha radiation is biologically much more damaging than 1Gy or 100R of x- or gamma radiation a quality factor is used. The dose in Sieverts or Rem is then equal to the dose in Grays or Rads multiplied by the quality factor, 1 Sievert or 100 Rem of any type of ionising radiation has the same biological effect.

Rad

Old unit of radiation absorbed dose. SI equivalent is the Gray.

Radiograph

Photographic image produced by a beam of penetrating ionising radiation which has passed through an object.

Radiographic contrast

Contrast in radiograph; usually expressed in terms of density difference. The ability of a combination of radiograph and human eye to differentiate between two areas of different subject thickness. Radiographic contrast is the combined effect of subject contrast and film contrast.

Radiographic exposure

Subjection of an emulsion to radiation to produce a latent image; commonly expressed in milliampereminutes or curie-hours.

Radiographic range (latitude)

Maximum range of thickness of a specified homogeneous material which can be recorded satisfactorily in a single radiograph with a specified technique.

Reciprocity law

Law which states that, all other conditions remaining constant, the time of exposure required to produce a given density is inversely proportional to the intensity of the radiation (see Bunsen-Roscoe Law).


9


Rem

Roentgen equivalent mammal, old unit of man mammal equivalent absorbed radiation dose. 1 Roentgen of alpha radiation has a much greater biological affect than 1 Roentgen of x- or gamma rays whereas 1Rem has the same biological affect whatever the type of ionising radiation. Despite the name the Rem is arrived at by multiplying the dose in rad (radiation absorbed dose) by a quality factor. SI equivalent is the Sievert.

Resolution

Smallest distance between recognisable images on a film or screen. It may be expressed as the number of lines per millimetre which can be seen as discrete images.

Reticulation

Effect due to rupture of an emulsion coating, usually caused by a rapid change of temperature. It gives an appearance similar to the grain of leather.

Rod-anode tube

Type of uni-polar (grounded anode) x-ray tube in which the target is near the end of a long tubular anode.

Roentgen

Old unit of exposure or ionising effect. SI equivalent is the coulomb per kilogram.

Salt screen

Intensifying screen consisting of a material such as calcium tungstate, which fluoresces in the visible or ultraviolet region of the spectrum under the action of ionising radiation. Seldom used in industrial radiography.

Sand casting

Casting process where a molten charge is poured into a mould formed from compressed sand, is the most versatile of all the casting processes, but suffers from coarse grain structure and poor surface finish.

SAW

Submerged arc welding. Automatic or semiautomatic arc welding process that offers deep penetration and a high deposition rate. A consumable wire is reel fed and the arc is struck under a layer of powdered flux.

Scattering

Redirection of radiation, with or without a change in photon energy (but usually with a reduction in photon energy), during its passage through matter.

Screen-type film

X-ray film designed for use with salt screens. It is sensitive to the fluorescent light emitted by such screens under the action of x-rays.

Secondary radiation

Radiation, other than primary radiation, emerging from the absorber.

Sievert

SI unit of man/mammal equivalent dose, abbreviation: Sv.


10


Source-to-film distance

Distance from a source of radiation to a film set up for a radiographic exposure, abbreviation: SFD.

Specific activity

Amount of radioactive material per unit mass of a sample, usually expressed in Curies/gram.

Speed

Relative rate at which a photographic emulsion reacts to exposure to radiation.

Standard temperature and pressure (STP)

The conditions most often used in chemistry to study or test a chemical. STP is 0 °C ( 273.15 Kelvin) and 100 kPa (1 bar or 0.986 atm) of pressure.

TIG

Tungsten inert gas welding. Manual or fully automatic arc welding process, extremely versatile but requires a high degree of operator skill. The heat source is an arc struck under a shield of argon or helium gas between a non-consumable tungsten electrode and the work piece. Filler wire may be fed into the arc, or in some circumstances a weld may be produced without filler wire.


11


Appendix

Appendix: Ionising radiation regulations Ionising Radiation Regulations 1999 (SI 1999 N. 3232) - Summary Details Requirements Controlled area A controlled area is one to which access is restricted due to the possibility of high radiation dose rates. Controlled areas must have well signed physical boundaries. Flashing lights shall be used to warn of an exposed source or energised x-ray tube. Flashing lights and a clear audible warning shall be used to warn of imminent exposure of a source or energising of an x-ray tube. Access to a controlled area is generally limited to classified personnel, but other persons may enter under strict supervision. Supervised area A supervised area is one in which the ionising radiation dose is likely to significantly exceed natural background radiation levels. There are no specific requirements for signposting supervised areas. Physical barriers are not required. Persons should not be allowed to linger in, but they are not restricted from passing through a supervised area. Classification of personnel Personnel who are likely to receive a significantly higher than normal ionising radiation dose in the course of their work duties should be classified. Classified personnel have their exposure to radiation monitored by an approved dosimetry service (eg the NRPB); in addition some employers operate their own internal dose monitoring schemes. Employers are required to investigate where the recorded dose of any classified worker exceeds 15mSv in one year; employers are encouraged to set their own investigation level some way below 15mSv per year. All classified personnel shall annually be declared fit by the appointed doctor. The appointed doctor may carry out a full medical or he may consider it sufficient to base this declaration on dose records alone. There is no specific requirement for blood tests; such things are at the discretion of the appointed doctor.

NDT20-71015 Appendix: Ionising radiation regulations

1

All areas where the annual radiation dose is expected to exceed 6mSv shall be designated as controlled areas. The radiation dose rate at the boundary of a controlled area shall not exceed 7.5μSv/h.

All areas where the annual radiation dose is likely to exceed 1mSv shall be designated as supervised areas. The radiation dose rate within a supervised area shall not exceed 7.5μSv/h.

All personnel age 18 or over who are likely to exceed an ionising radiation dose of 6mSv per year shall be classified. All radiographers shall be classified. A person under the age of 18 cannot be classified and cannot work as a radiographer. Classified personnel shall be declared fit by the appointed doctor on an annual basis. The recorded whole body dose of a classified worker shall not exceed 20mSv. A formal investigation is required if the recorded whole body dose for a classified person exceeds 15mSv in any period of one year.


Trainees The annual ionising radiation whole The annual dose of a trainee under the age of body dose for a trainee under the 18 must not exceed the stated limit of 6mSv, age of 18 shall not exceed 6mSv. but it is likely to be higher than that of a A person under the age of 18 cannot general member of the public. Typically a be classified trainee would be spending a higher than average amount of time within a supervised area. General public The annual ionising radiation whole In setting the dose limit for members of the body dose for a member of the public (and other mammals) it is taken into general public shall not exceed consideration that such members of the public 1mSv. (or other mammals) could be pregnant. A developing foetus is particularly sensitive to ionising radiation. Annual dose limitations – summary (SI 1999 n0. 3232 schedule 4) Description Classified worker Trainee General public Whole body 20mSv (1.) (2.) (3.) 6mSv 1mSv(4.) Lens of the eye 150mSv 50mSv 15mSv Skin 500mSv 150mSv 50mSv Hands, forearms, 500mSv 150mSv 50mSv feet, ankles Notes  Where special circumstances apply – an employer is able to show that the annual limit of 20mSv is impractical, up to 50mSv can be received in a single calendar year but not more than 100mSv over any five year period. Any employee exceeding these limits is likely to be suspended from work pending an investigation by the HSE.  Where the person in question – note 1 above, is a pregnant female the dose shall not exceed 13mSv in any period of 3 months.  Where the recorded dose exceeds 20mSv in one year the employer is required to make a formal investigation to determine whether the dose limitations of note 1 are likely to be complied with. The employer must report the matter to the HSE and must put in place a programme to ensure that the dose limitations of note 1 are not exceeded.  For persons who act as carers to others who receiving exposure to ionising radiation for medical purposes the dose limit is 5mSv in any five year period. Annual dose limitations – general. The dose limitations, items 1 to 6 above are additional to exposure to radiation for medical purposes. Radiation monitors Radiation monitors must be checked before use Radiation monitoring equipment shall be properly maintained. to ensure correct functioning; typically this Radiation monitoring equipment would involve a battery check and a check to shall be fit for the designated see that a reading is produced when the purpose. instrument is exposed to a source of ionising Radiation monitoring equipment radiation. shall be calibrated at appropriate Radiation monitors must have a scale intervals. appropriate to the magnitude of the doses being measured. General good practice is to have portable monitors calibrated on an annual basis. Appropriate calibration periods can vary dependent on the type and usage of the radiation monitor in question.


2


Radiation protection adviser Usually a person, but may be an organisation, meeting the HSEs criteria of competence. In general the RPA must be fully aware of the company’s activities involving ionising radiation. The local rules should be approved by, if not written by the RPA. The RPA is often called upon to undertake training of the radiation protection supervisors. Radiation protection supervisor A person appointed by the employer and named in the local rules to act as such. The RPS has in depth knowledge of the local rules. The RPS would generally take control where an emergency situation occurs and carry out initial investigation of any recorded or suspected overdose. General good practice requires the presence of at least one RPS where radiography is performed at an on-site location. Appointed doctor A registered medical practitioner appointed in writing by the HSE. Approved dosimetry service An organisation, approved in writing by the HSE, which monitors and records the ionising radiation doses of classified personnel. Minimum notification period Under most circumstances all work involving the use of ionising radiation must be notified to the HSE. Local rules The local rules are employer specific and describe in detail how the employer will control ionising radiation work such that the ionising radiation regulations are fully complied with.


3

All organisations working with ionising radiation shall appoint a radiation protection adviser.

All companies working with ionising radiation must appoint at least one RPS. The names and contact details of all RPS shall be listed in the local rules

Once a year all classified personnel shall be certified fit by an appointed doctor. All companies employing classified personnel must contract out their dose monitoring to an approved dosimetry service company. The minimum notification period is 28 days prior to the planned commencement of work. A copy of the local rules must be present at all work sites. The local rules must be regularly updated to reflect regulatory changes, changes in working conditions and changes in personnel.


Course Tips

 Read your notes carefully.  Make notes during the lecture on the paper provided in your folder.

Radiographic testing (RT) Welds NDT20

 Complete all homework given these are questions that may arise in your exam.  Ask questions if you are not sure the lecturer will do his/her best to explain further. Copyright © TWI Ltd


1

Course Objectives


 To explain the basic theory of X- and gamma radiography.  To select film type and energy levels, select and prepare techniques for a given specimen.  To state the theory of film processing and carry out practical dark-room work.  To have a working knowledge of basic radiation safety.  To plot and evaluate film characteristics (sensitometry).  To recognise film faults.  To meet the syllabus requirements for CSWIP/PCN Level 2.



History of Radiography W C Roentgen 1895  Discovered X-rays during another experiments with tube containing an anode and cathode.  The material around tube fluoresced and nearby photographic plates fogged.

Part 1: Theory Covering pages 1-15 of your notes



History of Radiography


Henri Becquerel 1896

Marie Curie 1898

 Discovered gamma-rays whilst working with fluorescent minerals. After storing a uranium compound in his drawer with some photographic plates he discovered were fogged.

 Discovered the radioactive sources Polonium and Radium. Radium was the first gamma source used for industrial radiography.  In 1946 man-made sources were produced including Cobalt and Iridium.



1

History of Radiography William Coolidge 1920’s


First radiograph 22 December 1895. Sent to Physicist Franz Exner in Vienna. (Mrs. Roentgen's hand.)

 Invented the X-ray tube as we know it today which revolutionised industrial radiography.

First x-ray tube belonging to Roentgen in 1896.

 He was awarded 83 patents due to his invention.


Radiographic Inspection


Radiographic Inspection

 X-rays are capable of passing straight through a solid object.  The amount of X-radiation that passes through a given object depends on the density and thickness of the object.  Transmitted X-radiation can be detected by photographic film or fluorescent screens.  This forms the basis of radiographic inspection, a powerful technique which is applicable to virtually all materials.


Advantages of Radiography  Directly produces a permanent record.  Capable of detecting internal flaws.  Useful for the non-destructive testing of virtually all materials and product forms.  Real-time imaging is possible in some applications.



Disadvantages of Radiography  Radiation hazard.  Sensitivity is affected by defect orientation.  Limited ability to detect fine cracks and other planar defects.  Access to two sides is required.  Limited by material thickness.  Skilled interpretation is required.  Relatively slow.  High capital outlay and running costs.


2

What is radiation?

Electromagnetic Spectrum

 Radiation  Radio waves, light, X-rays, g-rays and other forms of radiation take the form of waves of energy associated with electrical and magnetic fields which are at right angles to each other and the direction of propagation.  Electromagnetic radiation  Electromagnetic radiation has no mass and is not affected by magnetic or electrical fields nor to any great extent by gravity.


Wavelength v Photon Energy


Shorter Wavelength = Increased Energy

 

V f

V  2 . 997 x 10

8

m / sec

E = hf Where h is planks constant (= 6.626196 x 10-34Js)


Properties of Electromagnetic Radiation     

Travels at the speed of light. Travels through a vacuum. Travels in a straight line. No electrical charge or mass. Intensity proportional to 1/D² where D is the distance from the source.



Inverse Square Law

D1 D2


3

Properties of Electromagnetic Radiation X and gamma rays Absorbed and scattered by matter. Not refracted by matter. Cause ionisation referred to as ionising radiation.  Will darken photographic film emulsion and will cause some materials to fluoresce in the visible or ultraviolet spectrum.  Cannot be detected by human senses.  Extremely hazardous to health.    

Atomic Structure  Atoms are thought to consist of a positively charged nucleus surrounded by one or more negatively charged electrons that orbit the nucleus.  The nucleus consists of positively charged particles called protons and electrically neutral particles called neutrons.  A neutron can be thought of as a proton closely combined with an electron.


Atomic Structure  Protons have positive charge and by definition an atomic mass of 1.  Neutrons have no electrical charge, atomic mass very slightly greater than 1.  Electrons have negative charge equal in magnitude to that of a proton but are very much smaller at a mass of 1/1836 of a proton.  Number protons equal number electrons, usually!


Atomic Structure The nucleus of the atom is made up of protons and neutrons. The electrons orbits the nucleus. Proton: Positive charge. Neutron: No charge. Electron: Negative charge. A neutron can be thought of as a proton(+) with an electron(-) tightly attached.


Atomic Structure

HYDROGEN


Atomic Structure

Helium Atom 2 PROTONS

1 PROTON

2 ELECTRON

1 ELECTRON

2 NEUTRONS

No charge

Positive charge: Ionisation has occurred



4

Atomic Structure

LITHIUM

Atomic Structure

BERYLIUM

3 PROTONS 4 PROTONS

3 ELECTRONS 4 NEUTRONS

4 ELECTRONS 5 NEUTRONS


Atomic Structure


Atomic Structure: Helium



Atomic Structure: Hydrogen


Atomic Structure


5

Atomic Structure

Isotopes

 Atoms of an element having the same atomic number but different atomic mass.  The difference in atomic mass is due to a difference in the number of neutrons in the nucleus.  Some isotopes are stable while others are unstable.


Isotopes of Hydrogen


Deuterium

DEUTERIUM 1 PROTON 1 NEUTRON 1 ELECTRON




Tritium

TRITIUM 1 PROTON


2 NEUTRONS 1 ELECTRON



6

Radioactive Isotopes

 Some isotopes are stable, others are not.  Unstable isotopes transform into another element and in so doing emit radiation.  Three forms of radiation: 1. Alpha . 2. Beta . 3. Gamma .  Neutrons may also be emitted.

Radioactive Emissions Alpha particles: +  Emitted by large nuclei such as uranium or plutonium.  Composed of two protons and two neutrons with a helium nucleus.

226 88

4 Ra  222 86 Rn  2 He


Radioactive Emissions Beta particles:  Emitted by neutron rich nuclei such as uranium or plutonium.  Composed of high speed electrons.

14 6

Radioactive Emissions Gamma particles or photons:



 Emitted following the emission of an alpha or beta particle.  Composed of photons of energy not particles.

C  147 N  e  Copyright © TWI Ltd

Radioactive Decay

210 82


210 206 Pb  210 Bi  Po  83 84 82 Pb

 




Rate of Decay  Curie: 3.7 x 1010 disintegrations/second.  Becquerel: 1 disintegration/second.  Half Life: Time taken for the activity of an isotope to reduce by a half. Cobalt 60: 5.3 years. Iridium 192: 74 days. Ytterbium 169: 32 days. Uranium 238: 4.47 x 1010 years. Selenium 75: 118 days

  Copyright © TWI Ltd


7

Rate of Decay

Rate of Decay

 At the level of individual atoms radioactive decay is random but for each isotope each individual atom has the same probability of decay.  A 2 x 1mm cylinder of Iridium contains around 1020 atoms; when so many atoms all have the same probability of decay the result is a constant half life even though each individual decay event is random.



Industrial Radiography

 X - Rays  Electrically generated.

 Gamma Rays  Generated by the decay of unstable isotopes.


8

Radiographic Testing (RT) Welds NDT20

Part 2: Equipment Covering pages 16-40 of your notes


X-Ray and Gamma Equipment


X-Ray Production  X-rays are produced by the deceleration of high velocity electrons.  Part or all of the kinetic energy of the electron is converted into electromagnetic radiation (X-rays).  Kinetic energy is controlled by velocity: Ek = ½mv2 and is usually stated in keV or MeV.  Electron velocity in an X-ray tube is controlled by tube voltage.

X-Ray Production


X-Ray Production    

Requirements Electron source. Means of accelerating electrons to a high velocity. Means of halting electrons.



X-Ray Production 1. Electron source: Tungsten filament


1

X-Ray Production

X-Ray Production 1. Electron source: Tungsten filament

1. Electron source: Tungsten filament

Current

Current

Free electrons

Thermionic emission of electrons



X-Ray Production 2. Accelerating electron: Potential difference

-ve -ve

+ve


+ve

-ve




-ve

+ve

Focusing cup concentrates electrons into a beam


X-Ray Production 3. Means of halting electrons: High density material

-ve

+ve

Tungsten target


2

X-Ray Production 3. Means of halting electrons: High density material

-ve

+ve

X-Ray Production Kinetic energy converted to heat and x-rays

-ve

+ve

X-rays / Bremsstrahlung


Problems


X-Ray Production

 Electrons travel for only short distances through gasses.  Kinetic Energy converted into ± 95 % heat and ± 5 % X-rays.


X-Ray Production: HEAT  In any x-ray tube around 95 % of the energy generated is in the form of heat.  For typical 200 kV portable equipment around 1 kW of heat has to be dissipated.  For a 300 kV constant potential laboratory unit heat generation is typically 7.5 kW.  X-ray tubes of all types therefore require a cooling system in order to prevent overheating and increase duty cycle.  Older type sets having glass envelope tubes are generally oil or gas cooled.



X-Ray Production: HEAT  A rotating anode may be used in order to help dissipate heat. This type of arrangement is generally limited to X-ray units intended for medical use.  Modern X-ray units have so-called metal-ceramic envelopes. The use of such envelopes makes it practical to have a much higher potential difference between the electrodes and the envelope than was the case with glass.  This in turn permits the use of grounded anodes.  Such anodes are at zero volts and can therefore be cooled directly by water.


3

X-Ray Production: Anodes Directional Type Hood

X-Ray Production: Anodes Panoramic Type Beryllium window

Tungsten target Tungsten target

Heavy high conductivity Copper heat sink

Beryllium window

Hood

Heavy high conductivity Copper heat sink



X-Ray Production: Anodes Rod-anode

X-Ray Production: Anodes

Rotating-anode Tungsten target

Used mainly for low kV, very high tube current, equipment in medical applications.

Beryllium window

Aluminium tube Copyright © TWI Ltd


 Tube voltage controls the quality or penetrating ability of the radiation.

X-ray tube

HT transformer

Rheostat

mA kV


LT transformer

 Tube current controls the amount or intensity of radiation.

X-Ray Production

Autotransformer

X-Ray Production

AC Power

AC Power


4

X-Ray Production  Current can flow across an X-ray tube only when the cathode (i.e. the filament) is negative and the anode (i.e. the target) is positive.  Therefore if an X-ray tube is energised using a simple AC supply X-rays will be produced only when the supply polarity is such that the cathode is negative and the anode is positive.  Simple AC X-ray machines are therefore referred to as self-rectified.

X-Ray Production  Output of C-rays can be more than doubled if the AC supply is rectified.  X-ray equipment fitted with a rectifying circuit is referred to as constant potential.  Most CP units use a Greinacher circuit to rectify the AC supply.  CP units produce harder radiation than SR even when operating at the same tube voltage.



X-Ray Production

X-Ray Production

Decreasing energy

Radiation intensity

Radiation intensity

Decreasing energy

X-Ray Production

Increasing wavelength Copyright © TWI Ltd

X-Ray Production

Increasing wavelength


X-Ray Production

Radiation intensity

Decreasing energy

Increasing wavelength



5

X-Ray Production

High Energy X-Ray Sources  X-Ray energies of up to 30 MeV are produced using linear accelerators or betatrons. Electrostatic (Van der Graaf) generators are also used occasionally.  Linear accelerators (Linacs) accelerate electrons to high velocity using an electric wave (RF). Electrons surf the electrical waves and attain high velocity.  Betatrons accelerate electrons along a spiral path by means of magnetic fields.  Van der Graaf generators can develop high electrical potentials by mechanical means such electrical potentials can be used to accelerate electrons.



X-Ray and Gamma Equipment

Sealed Sources

Gamma rays



Gamma-Ray Equipment

Gamma-Ray Equipment Projection tube

Isotope container

Wind-out Copyright © TWI Ltd


6

Isotopes Used in Industrial Radiography Half life

mm of steel

Iridium 192

74.4 days

20 - 100

Cobalt 60

5.3 years

40 - 200

Ytterbium 169

32 days

1 - 15

Selenium 75

119 days

10 - 40

Thulium 170

128 days

up to 5

Caesium 137

30 years

20 - 80

Relative intensity/% of total

Isotope

Isotopes Used in Industrial Radiography

Radiation energy/MeV



Isotopes Used in Industrial Radiography Relative intensity/% of total

Relative Intensity/% of total


Radiation energy/MeV

Radiation energy/MeV Copyright © TWI Ltd


Isotopes Used in Industrial Radiography Relative intensity/% of total

Relative intensity/% of total




7

Advantages Gamma rays compared with X- rays  No water or electrical supplies needed.  Equipment smaller and lighter therefore more portable.  Easier to perform radiography in confined or difficult to access areas.  Equipment simpler and more robust.  Less scatter no low energy radiation.  Less initial cost.  Greater penetrating power.


Disadvantages Gamma rays compared with X- rays  Reduced radiographic contrast.  Exposure times generally longer.  Sources need replacing potentially greater inservice costs.  Radiation cannot be switched off.  Generally inferior geometric unsharpness, SFD is usually minimised to obtain economic exposure time.  Remote handling necessary.  Penetrating power cannot be adjusted. Copyright © TWI Ltd

8


Part 3: Image processing Covering pages 40-48 of your notes



Radiographic Film

Radiographic Film

Subbing

Base

Base Subbing



Radiographic Film

Radiographic Film Supercoat Emulsion mainly AgBr in gelatine base

Emulsion mainly AgBr in gelatine base

Subbing

Subbing

Base

Base

Subbing

Subbing Emulsion mainly AgBr in gelatine base

Emulsion mainly AgBr in gelatine base

Supercoat



1

Radiographic Film Pre-exposure

After exposure

Latent Image Formation Bromine ion BrSilver Ion Ag+ Interstitial Silver Ion Ag+ Free electron Silver atom Ag

Un-sensitised: Stable

Sensitised: Unstable

During exposure a latent image is formed by sensitised Silver Halide crystals.


Latent Image Formation  Silver bromide crystals are not perfect they contain interstitial silver ions.  When an interstitial silver ion accepts a free electron it becomes a silver atom.  The silver atom is larger than the ion and exerts a stress on the crystal lattice.  In the presence of developer this stress causes instability and the crystal breaks down resulting in the whole of the crystal changing to black metallic silver.


Film Types

Grain Size Speed

Quality

Coarse

Fast

Poor

Medium

Medium

Medium

Fine

Slow

Good

Ultra Fine

Very Slow

Very Good


Latent Image Formation  The interstitial silver atoms nucleate silver crystals.  A single interstitial silver atom is sufficient to cause an entire silver bromide crystal to convert to metallic silver.  The typical size of a silver bromide crystal in a typical photographic film emulsion is about 1 μm.  Sensitisation of a silver bromide crystal can be caused by just a single photon of X-ray energy.


Intensifying Screens  Film is usually placed between front and back intensifying screens.  Generally lead of 0.03 - 0.15 mm occasionally salt screens may be used.  Lead screens shorten exposure time and improve image quality by helping to reduce the effects of scattered radiation.  Salt screens shorten exposure time, often dramatically but produce inferior image quality.



2

Intensifying Screens  Metallic: Usually lead but other metals such as copper may be used.  Salt: Usually calcium tungstate.  Fluorometallic: These are salt screens with a metal foil backing. They combine the advantages of metallic and salt screens, however, they are extremely expensive and they are easily damaged.

Intensifying Screens: Metallic  For radiation energy of 120 keV or greater front and back lead intensifying screens are commonly used.  The optimum thickness of such screens varies with radiation energy but 0.03 - 0.15 mm is typical.  The front screen reduces the effect of radiation scattered by objects situated in front of the film including the object which is being radiographed and helps to shorten exposure time.  The back screen reduces the effect of radiation scattered by objects situated behind the film and to a lesser extent when compared with the front screen helps to shorten exposure time.



Intensifying Screens

Front screen Front emulsion

Intensifying Screens

Primary radiation Secondary electrons

No screens

Pb screens: Poor contact

Grain sensitised by primary radiation

Base Back emulsion

Grain sensitised by secondary electrons

Back screen

Pb screens: Good contact



Intensifying Screens  Metal usually Pb: Intensification factor about 2x for radiation energies in excess of 120 keV.  Salt: Intensification factor may be as high as 500x.  Fluorometallic: Intensification factor about 50x.


Film Processing     

Developer: Reducing agent: Alkaline. Stop bath: Acetic acid. Fixer: Dissolves silver halide: Acidic. Washing. Drying.


3

Film Processing  Development  Latent image converted into metallic silver in 3-5 minutes at 20°C.  The four main constituents of developer:  Reducing agent: Metol/hydroquinone.  Accelerator: Keeps solution alkaline.  Restrainer: Ensures only sensitised silver halides converted.  Preservative: Prevents oxidation by air.

Developer  In order to increase the working life of the developer replenisher should be added in accordance with manufacturer’s recommendations.  Replenisher replaces used reducing agent and maintains alkalinity. It also maintains the depth of developer in the processing tank, during processing there are losses due to carry-over and evaporation.  Keeping a record of how much film has been processed helps in deciding how much replenisher to add.



Film Processing Stop bath

3% acetic acid neutralises the developer, stops the development process and increases fixer life. It is common to add an indicator to the stop bath to confirm it’s acidity. The stop bath should always be held at approximately the same temperature as the developer and fixer as sudden temperature changes can damage the film emulsion. A soaking time in the stop bath of just a few seconds is sufficient to arrest development and neutralise alkalinity.

Film Processing Fixer

A solution of sodium thiosulphate or ammonium thiosulphate. Fixer is mildly acidic, acetic acid stabilises the solution. Unexposed undeveloped silver halides are leached out of the film emulsion. Fixer commonly contains a hardener. This helps to promote rapid even drying it also makes the wet film easier to handle. Fixing time is generally taken to be twice the clearing time. Leaving film in fixer for an extended period may cause the film emulsion to peel away from the base.


Fixer


Fixer

Like developer fixer may be replenished. When unexposed film is placed in the fixer bath it will be observed to clear as the silver halides are dissolved by the fixer. Clearing time is an important measure of fixer condition. Clearing time for new fixer will usually be less than 30 seconds. Radiographs are usually fixed for a time equal to twice the clearing time.

During the fixing process large amounts of silver accumulate in the fixer solution. Therefore it is common practice to recover silver from spent fixer.

Fixer contains a hardener which helps to prevent swelling of the film emulsion and accelerate drying.



4

Film Processing

Advanced Imaging Techniques Computed radiography

Washing

20-30 minutes in clean running water. Usually followed by dipping in a clean water bath containing a wetting agent which helps to promote even drying. Note: Over washing must be avoided.

Optical scanner

Laser beam A/D converter

Over washing will cause swelling and excessive softening of the film emulsion a major cause of drying marks. Insufficiently washed radiographs will discolour and their shelf life will be limited.

Photo-multiplier tube

Imaging plate

110010010010110

Motor Copyright © TWI Ltd

Image Software Image (1)


Real Time Radiography

Edge detection (2)

Subtraction of (2) from (1)


Real Time Radiography


Computed Tomography

 Image type  The image formed is a positive image since brighter areas on the image indicates where higher levels of transmitted radiation reached the screen.

Positive image

Negative image Copyright © TWI Ltd


5

Computed Tomography

1

3

2

4

As the component is rotated a series of 2D slices are collected when these are superimposed and merged using computer software according to the radial position a 3D image is then produced.

CT Images  As shown below the 3-D image can then be manipulated and sliced in various ways to provide a thorough understanding of the structure and nature of defects.

The cross section of the test piece becomes more defined as it is rotated the stretched density information is added to the image.



6


Part 4: Image quality Covering pages 48-65 of your notes



Radiographic Sensitivity

Factors Influencing Sensitivity

The ability of a radiograph to detect a small change in section thickness Affected by:  Definition: The degree of sharpness of a radiographic image.  Contrast: The degree to which two adjacent areas of different film density can be distinguished one from the other.

Sensitivity

Contrast

Definition





Sensitivity

Contrast Film density

Film type

Sensitivity

Definition Quality of radiation

Subject contrast

Film processing

Contrast Film density

Agitation


Film type

Development time

Definition Quality of radiation

Developer temperature

Subject contrast

Film processing

Developer concentration

Type of developer


1



Sensitivity

Contrast Type of film

Sensitivity

Definition

Intensifying Radiation Relative Geometry Film screens quality movement processing

Contrast Type of film

Agitation

Definition

Intensifying Radiation Relative Geometry Film screens quality movement processing

Development time

Developer Developer temperature concentration



Image Quality  Contrast  The ability to differentiate areas of different film density


Radiographic Quality

Type of developer

Radiographic Contrast  Insufficient Contrast  kV too high.  Over exposure compensated for by shortened development.  Incorrect film: Screen combination.  Scatter.  Fogged film.  Poor film processing.

 Excessive Contrast  kV too low.  Incorrect developer.  Incorrect film: Screen combination.


Geometric Unsharpness

Definition

The sharpness of the dividing line between different density fields. There are two types of unsharpness in radiography: 1. Film or inherent unsharpness. 2. Geometric unsharpness or penumbra.



2


Geometric Unsharpness Long Film to Object Distance

=

Low Ug





Small focus

Short Film to Object Distance

High Ug

Low Ug


Geometric Unsharpness Large Focus



Short Object to Film Distance

Low Ug

High Ug



3


Reducing Geometric Unsharpness    

Long Object to Film Distance

Source size as small as possible. Source to object distance as long as practical. Object to film distance as short as possible. In a good quality technique geometric unsharpness should be not more than the inherent unsharpness of the film screen combination in use.

High Ug



Measuring Geometric Unsharpness  The drilled holes in step: Hole and plaque type IQIs are intended as an indicator of geometric unsharpness.  Standard wire type IQIs (EN ISO 19232-1) are poor indicators of geometric unsharpness.  A special type of IQI called a duplex wire IQI is designed with a view to measuring geometric unsharpness (EN ISO 19232-5).

Duplex IQIs  EN ISO 19232-5 defined duplex wire IQI containing 13 pairs of circular cross-section wires made of platinum and tungsten.

Note: EN 462-5 was former standard which is technically identical with EN ISO 19232-5 Copyright © TWI Ltd


BS 3971 Duplex IQI

Duplex identification D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13

Wire diameter [mm] 0,80 0,63 0,50 0,40 0,32 0,25 0,20 0,16 0,13 0,10 0,08 0,063 0,05

Achieved basic spatial resolution [mm] 0,80 0,63 0,50 0,40 0,32 0,25 0,20 0,16 0,13 0,10 0,08 0,063 0,05

BS 3971 Duplex IQI

Achieved geometrical unsharpness

[mm]

1,60 1,26 1,00 0,80 0,64 0,50 0,40 0,32 0,26 0,20 0,16 0,13 0,10


First unresolved duplex is ... Duplex identification D7 D8 D9 D10

Wire diameter [mm] 0,20 0,16 0,13 0,10

Achieved basic spatial resolution [mm] 0,20 0,16 0,13 0,10

Achieved geometrical unsharpness

[mm]

0,40 0,32 0,26 0,20


4

Inherent Unsharpness Inherent or film unsharpness is due to:  The graininess of the film, fast films have larger grain size than slow films.  The type of intensifying screens, metallic foil screens are much better than fluorescent screens.  The radiation energy, film unsharpness is increased at high radiation energy.  Film processing, development time and temperature affect grain size.


Scattered Radiation

Scattered radiation can seriously effect the quality of a radiographic image and needs to be considered with a view to reducing its effect on the final image quality.


Scatter Three major causes of scatter are:  Photoelectric effect.  Compton scattering (incoherent scatter).  Pair production.  Other scattering mechanisms exist for example: Rayleigh scattering (coherent scattering).


Inherent Unsharpness Actual object Ultrafine grain film Fine grain film Coarse grain film Copyright © TWI Ltd

Scatter  Radiation originating from any source other than the primary source.  Primary radiation is absorbed then re-emitted in all directions.  Scatter is a major contributor to poor radiographic contrast and definition.  Scatter may also cause a radiation hazard, dose rates maybe locally increased by scattering effects.


Photoelectric Effect  In the photoelectric effect an electron absorbs all of the energy of the incident X-ray photon.  If the photon energy is sufficient the electron will be completely ejected from the atom and ionisation will occur.  Where the incident photon has exactly the right amount of energy the electron may simply jump from one energy level to another.  As the affected atom returns to its base state low energy X-rays are emitted in all directions.


5

Photoelectric Effect

Compton Scattering  In Compton scattering an electron absorbs part of the energy of the incident X-ray photon.  In Compton scattering the affected electron is ejected from the atom and ionisation results.  The photon energy not absorbed by the electron is deflected from the original path of the incident photon as an X-ray of lower energy.  As the affected atom returns to its base state low energy X-rays are emitted in all directions.


Compton Scattering


Pair Production  In pair production a high energy X-ray photon converts to an electron, positron pair following interaction with either an orbital electron or an atomic nucleus.  Pair production occurs only above a threshold energy of 1.02 MeV.  A positron has a very short life expectancy, it quickly interacts with an electron causing annihilation of both particles.


Pair Production


Pair Production

 Annihilation of an electron, positron pair produces scattered radiation at a characteristic photon energy of 0.51 MeV.  The electron produced in the pair production event has high velocity and causes ionisation and further production of scattered radiation.



6

Coherent (Rayleigh) Scattering

Coherent (Rayleigh) Scattering

 In coherent scattering there is no loss of photon energy.  The incident photon is effectively deflected from its original path as it interacts with an atom.  The incident photon is momentarily absorbed by the atom setting its orbital electrons in oscillation, then re-emitted without energy loss but in a new direction.  Coherent scattering affects only very low energy X-ray photons and is of little importance in industrial radiography. Copyright © TWI Ltd

Mode of Scatter vs Radiation Energy  At radiation energies up to approximately 600 eV the photoelectric effect is the dominant scattering mechanism.  From 600 eV to approximately 6 MeV Compton scattering predominates.  Above 6 MeV pair production takes over as the dominant effect.  The total amount of scattering as a proportion of the incident radiation is much greater at energies below 1 MeV than it is at higher energies.


Scatter  Internal scatter: Originating within the specimen.  Side scatter: Walls and nearby objects in the path of the primary beam.  Back scatter: Materials located behind the film.



Scatter

Scatter

 Internal scatter originating within the specimen

 Side scatter from walls and nearby objects in the path of the primary beam



7

Scatter  Back scatter materials located behind the film

Checking For Back Scatter British, European and American codes and standards describe a method of checking for back scatter.  A lead letter B is attached to the back of the film cassette during exposure.  If a light image of the letter B appears in the radiographic image then excessive back scatter is present and the radiograph must be retaken.  A dark image of B does not indicate backscatter!!!



Checking For Back Scatter Light image of B: Reshoot

Scattering Angle  The angle formed between the direction of the primary radiation beam and the direction of travel of the scattered radiation is referred to as scattering angle or angle of scatter.

Angle of scatter

Dark image of B: Accept Primary radiation Copyright © TWI Ltd


Scattering Angle

Scatter

 Scattered radiation with a scattering angle of less than or equal to 90 is side scatter or internal scatter.  Scattered radiation with a scattering angle of greater than 90 is back scatter.

Low scatter


High scatter


8

Control of Scatter        

Collimation. Lead screens. Protection from back scatter. Beam filtration (X-ray only). Blocking. Diaphragms. Grids (oscillating). Increased beam energy.

Inherent Unsharpness Typical inherent unsharpness for Pb screens/fine grain film Radiation source


Inherent unsharpness (mm)

100 kV X-rays

0.05

200 kV X-rays

0.09

300 kV X-rays

0.12

400 kV X-rays

0.15

1000 kV X-rays

0.18

Iridium 192

0.17

Cobalt 60

0.35


9

Radiographic testing (RT) welds NDT20

Part 5: Exposure calculation Covering pages 65-83 of your notes


Determining The Correct Exposure

Knowing how to determine the correct exposure to achieve the required radiographic film density is essential for those involved in radiographic testing. Incorrect calculation can lead to lost time caused by countless reshoots and the subsequent increase in film costs.


Image Quality Film density The degree of darkening of a processed film is called optical density. Optical density is a logarithmic unit:

Where I1 is the incident light intensity and I2 is the transmitted light intensity. Thus if film density = 2, the incident light intensity is 100 x greater than the transmitted intensity.


Radiographic Film Density National codes and standards for radiography of welds and castings invariably define a minimum level of film density:  ASME V requires a minimum film density of 1.8 for X-radiography of welds and a minimum of 2.0 for gamma techniques.  EN ISO 17636-1 requires a minimum film density of 2.0 for class A (basic techniques of X- or gamma radiography of welds) and a minimum of 2.3 for class B (improved techniques).



Radiographic Film Density  Radiographic films provide good contrast at film densities exceeding about 1.5.  Radiographs with a density exceeding 3.5 or perhaps 4.0 cannot be properly viewed and assessed on standard radiographic film illuminators.  Film density is easily measured using a densitometer or by comparison with a calibrated density strip.


1

Radiographic Film Density

Radiographic Film Density

Film density

Lack of density  Under exposure.  Developer temperature too low.  Exhausted developer.  Developer too weak.

Excessive density  Over exposure.  Developer temperature too high.  Excessive development.  Too strong a solution.



Factors Affecting Exposure

Exposure Charts Exposure/ mAmin 45 40 35 30 20 25

25 20

15

15

10

10 5

5

Steel Thickness / mm

30

Steel Thickness / mm

Film speed. Quality of radiation. FFD or SFD. Screens. Filters. Development. Density required. Intensity of radiation.

40 35

       

45

Radiographic

 Specimen  Material type.  Thickness.

Exposure / mAmin Copyright © TWI Ltd

Exposure Calculations Radiographic equivalence chart Radiation energy/isotope Material Steel

100keV

150keV 220keV

400keV

Ir192

1.0

1.0

1.0

1.0

1.0

Copper

1.5

1.6

1.4

1.4

1.1

Aluminium

0.08

0.12

0.18

-

0.35

Al alloy 4.5% Cu 0.13

0.16

0.22

-

0.35

Titanium

0.45

0.35

-

-

0.5


Equivalence Factors  Equivalence factors vary with radiation quality.  Equivalence factors are used to convert a thickness of a given material to a radiographically equivalent thickness of another material for which exposure times are known.  For example: Convert 10mm of steel to an equivalent thickness of copper using 100 kV X-rays: Te = 10 x 1.6 = 16 mm



2

Film Factors Classification ASTM E 1815

AGFA

C1

Special

D2

C2 C3

D3 Class 1

C4

 Film speed chart


EN ISO 11699-1

D4 D5

C5

Class 2

D7

C6

Class 3

D8

CF 9.0 4.2 2.6 1.6 1.0 0.7

KODAK

CF

FUJI

DR 50

7.2

IX 25

M 100

4.2

IX 50

MX 125

2.8

---

T 200

1.7

IX 80

AA 400

1

IX 100

CX

0.7

IX 150

CF

FOMA

6.5

R2

3.3

R3 R4

1.6

R5

1.0

R7

0.6

R8

Exposure Calculation

CF

D7

Agfa

D5

D4

9.0 4.2

Kodak

CX

AA 400

MX12 5

2.6 1.6 1.0 0.7

Fuji

IX IX IX 80 150 100 2 2.5 3 3.5 4 5 6 7 10 12 14 Relative exposure




New Exposure =

Original exposure: Original FFD: New FFD:

4 minutes. 0.7 2.8

4∙

2.8 0.7


Change of FFD

 Change of film: From CX to MX125. Original exposure: Film factor for CX: Film factor for MX:

8

16 minutes

New time = 4 ∙

4 minutes. 1000 mm. 750 mm.

750 1000

2.25 minutes


Characteristic Curves  Increasing exposures are applied to successive areas of a film.  After development the film density is measured.  The density is then plotted against the log of the relative exposure. The resultant graph is called the characteristic curve or sensitometric curve or Hurter-Driffield curve (H&D curve)


Characteristic Curves

Shoulder

Straight line section

Toe



3


The relationship between exposure time and resultant film density is non-linear. The gradient of the film characteristic curve is a measure of film contrast.


Film A is coarse grain and is faster than Film B and C. Film B is fine grain and it’s speed is intermediate between Film A and C. Film C is ultra-fine grain and is the slowest of the three. A fast film requires a shorter exposure time than a slow film.


Characteristic Curves 1.63 - 1.31 = 0.32 Antilog 0.32 = 2.1 Original exposure = 10 mAmin New exposure = 2.1 X 10 = 21 mAmin

Using D7 film a density of 1.5 was achieved using an exposure of 10 mAmin. What exposure is required to achieve a density of 2.5?


Image Quality Indicators  Image quality indicators (IQIs)


Characteristic Curves 2.07 - 1.63 = 0.44 Antilog 0.44 = 2.75 Original exposure = 10 mAmin New exposure = 2.75 X 10 = 27.5 mAmin

Using D7 film a density of 2.5 was achieved using an exposure of 10 mAmin. What exposure is required to achieve a density of 2.5 using MX film? Copyright © TWI Ltd

Image Quality Indicators Definitions  Radiographic sensitivity is the ability of radiographic system to reveal discontinuity of certain size on the radiographic image.

or  Penetrameters are used to measure radiographic sensitivity and the quality of the radiographic technique used.


 It can also be defined as a measure of quality of radiographic image. True radiographic sensitivity is difficult quantity to measure.


4

Image Quality Indicators Definitions  IQI sensitivity: Is not an exact measure of the true sensitivity of a radiographic technique.

Image Quality Indicators  EN ISO 19232-1 Wire type IQIs

 IQIs are used in radiography to ensure that the general overall quality of a radiographic technique is adequate.



Image Quality Indicators  EN ISO 19232-2 Step-hole type IQIs

Image Quality Indicators  ASTM E 1025 Plaque type IQIs

2T 1T

4T

2T 1T

XX: IQI thickness thousandths of an Plaque inch. T:

Sensitivity is measured in terms 2-2T where 2 equals a plaque thickness of 2 % of the test specimen thickness and 2T is the hole that is visible on the radiographic image.

thickness.


Image Quality Indicators  IQI sensitivity is usually expressed as a percentage of subject thickness.  For single wall single image and double wall single image techniques the single wall thickness is generally taken as subject thickness.  For double wall double image techniques the double wall thickness is used.



Determining The Correct Sensitivity

Sensitivity =

100 t T

T: Subject thickness. t: Thickness of thinnest discernible wire or step.


5

Image Quality Indicators  IQIs should wherever possible be placed on source side.  For the double wall single image technique this is not possible and IQIs are therefore placed film side.  Different requirements apply dependent on whether the IQI is source or film side.


6


Part 6: Techniques Covering pages 84-96 of your notes


Radiographic Techniques  Single wall single image (SWSI).  Double wall single image (DWSI).  Double wall double image (DWDI).


Radiographic Techniques

Technique: SWSI (panoramic) Required number of exposures: 1 Location marker placement: External IQI placement: External (followed by comparative exposure)





Technique: SWSI (source internal and offset)

Technique: SWSI (source external)

Required number of exposures: See EN ISO 17636-1 figures.

Required number of exposures: See EN ISO 17636-1 figures.

Location marker placement: External

Location marker placement: External

IQI placement: External

IQI placement: External



1


Radiographic Techniques Technique: DWDI (elliptical)

Technique: DWSI Required number of exposures: See EN ISO 17636-1 figures. Location marker placement: External IQI placement: External

Required number of exposures: See EN ISO 17636-1 figures. Location marker placement: Source side preferred IQI placement: Must be source side





Technique: DWDI (superimposed) Required number of exposures: See EN ISO 17636-1 figures. Location marker placement: Source side preferred IQI placement: Must be source side


Radiographic Techniques  Identification  Unique identification.


Radiographic Techniques  Identification  Unique identification.  Pitch markers: Location markers.



2

Radiographic Techniques Identification  Unique identification.  Pitch markers.  IQI’s.

Localisation  The through thickness position of a defect cannot be determined by single exposure radiography.  A technique called tube shift or source shift can be used to determine through thickness position.



Localisation

Using similar triangles:

= Therefore:

=t-


3

Radiographic Testing (RT) Welds NDT20

Part 7: Interpretation Covering pages 97-125 of your notes



Radiographic Interpretation

Welding Terminology  Butt joints

 In order to correctly interpret radiographs it is essential that the interpreter has a good knowledge of the product under examination and the possible defects that may arise due to various processes carried out on the test piece.

Square Edged

Closed

Open Single Sided Butt

Vee

Bevel Double Sided Butt

Vee



Welding Terminology  Fillet joints

Welding Definitions  BS 499-1  A union between pieces of metal at faces rendered plastic or liquid by heat pressure or both.

Lap Tee Corner


Bevel

 NASA  A continuous defect surrounded by parent material.


1

Welds An ideal weld must give a strong bond between materials with the interfaces disappearing

A union between pieces of metal at faces rendered plastic or liquid by heat, pressure or both.

BS 499-1

To achieve this:    

Welding

Smooth, flat or matching surfaces. Surfaces shall be free from contaminants. Metals shall be free from impurities. Metals shall have identical crystalline structures.

Possible energy sources  Ultrasonics.  Electron beam.  Friction.  Electric resistance.  Electric arc.


Electric Arc Welding Electrode Power supply Work piece Clamp (earth)


Electric Arc Welding  Electric discharge produced between cathode and anode by a potential difference (40-60 volts).  Discharge ionises air and produces -ve electrons and +ve ions.  Electrons impact upon anode, ions upon cathode.  Impact of particles converts kinetic energy to heat (7000oC) and light.  Amperage controls number of ions and electrons, voltage controls their velocity.


Electric Arc Welding


Zones in Fusion Welds  Parent material or base metal  Heat affected zone  Fusion zone

Arc welding processes:  Manual metal arc.  Tungsten inert gas.  Metal inert gas.  Submerged arc. Differences between them:  Methods of shielding the arc.  Consumable or non-consumable electrode.  Degree of automation.



2

Manual Metal Arc Welding  Shielding provided by decomposition of flux covering.  Electrode consumable.  Manual process.

    

Welder controls Arc length. Angle of electrode. Speed of travel. Amperage settings.

Manual Metal Arc (MMA) Consumable electrode Flux coating

Arc Evolved gas shield

Core wire

Slag

Parent metal

Weld metal Copyright © TWI Ltd


Tungsten Inert Gas (TIG)

Metal Inert Gas (MIG)

Gas nozzle Nonconsumable tungsten electrode

Filler wire

Arc

Parent metal

Gas nozzle

Consumable electrode (filler wire)

Reel feed

Gas shield

Gas shield

Arc

Weld metal

Parent metal

Weld metal



Submerged Arc

Submerged Arc Flux retrieval

Slag

Weld metal Copyright © TWI Ltd

Consumable electrode

Reel feed

Flux feed

Parent metal Copyright © TWI Ltd

3

Welding Defects Cracks Classified by shape  Longitudinal  Transverse  Branched  Chevron

Welding Defects

Cracks Classified by position  HAZ  Centreline  Crater  Fusion zone  Parent metal

Four crack types:  Solidification cracks.  Hydrogen induced cracks.  Lamellar tearing.  Reheat cracks.


Welding Defects


Welding Defects Solidification cracking

Cracks  Solidification  Occurs during weld solidification process.  Steels with high sulphur content (low ductility at elevated temperature).  Requires high tensile stress.  Occur longitudinally down centre of weld,  eg crater cracking.


Welding Defects Cracks


Welding Defects Hydrogen cracking

 Hydrogen Induced  Requires susceptible grain structure, stress and hydrogen.  Hydrogen enters via welding arc.  Hydrogen source, atmosphere or contamination of preparation or electrode.  Moisture diffuses out into parent metal on cooling.  Most likely in HAZ.



4

Welding Defects

Welding Defects Lamellar tearing

Cracks Lamellar tearing Step like appearance. Occurs in parent material or HAZ. Only in rolled direction of the parent material. Associated with restrained joints subjected to through thickness stresses on corners, tees and fillets.  Requires high sulphur or non-metallic inclusions.     

Restraint High contractional stress Lamellar tear



Welding Defects Cracks  Re-heat cracking  Occurs mainly in HAZ of low alloy steels during post weld heat treatment or service at elevated temperatures.  Occurs in areas of high stress and existing defects.  Prevented by toe grinding, elimination of poor profile material selection and controlled post weld heat treatment.

Welding Defects Incomplete root penetration Causes  Too large or small a root gap.  Arc too long.  Wrong polarity.  Electrode too large for joint preparation.  Incorrect electrode angle.  Too fast a speed of travel for current.



Welding Defects

Welding Defects Root concavity

Incomplete root fusion

Causes

Causes  Too small a root gap.  Arc too long.  Wrong polarity.  Electrode too large for joint preparation.  Incorrect electrode angle.  Too fast a speed of travel for current.

 Root gap too large.  Insufficient arc energy.  Excessive back purge TIG.



5

Welding Defects

Welding Defects

Excess root penetration Root undercut

Causes  Excessive amperage during welding of root.  Excessive root gap, poor fit up.  Excessive root grinding.  Improper welding technique.

   

Causes Root gap too large. Excessive arc energy. Small or no root face.



Welding Defects

Welding Defects

Cap undercut Lack of fusion Causes  Excessive welding current.  Welding speed too high.  Incorrect electrode angle.  Excessive weave.  Electrode too large.

Causes  Contaminated weld preparation.  Amperage too low.  Amperage too high (welder increases speed of travel). Copyright © TWI Ltd

Welding Defects Incompletely filled groove Lack of side wall Fusion


Welding Defects Inter run incompletely filled groove Causes  Insufficient weld metal deposited.  Improper welding technique.

 Causes  Insufficient weld metal deposited.  Improper welding technique.



6

Welding Defects Gas pores/porosity: Causes  Excessive moisture in flux or preparation.  Contaminated preparation.  Low welding current.  Arc length too long.  Damaged electrode flux.  Removal of gas shield.

Welding Defects Inclusions: Slag Causes  Insufficient cleaning between passes.  Contaminated weld preparation.  Welding over irregular profile.  Incorrect welding speed.  Arc length too long.



Welding Defects

Inclusions: Tungsten Causes  Contamination of weld caused by tungsten touching weld metal or parent metal during welding using the TIG welding process.

Welding Defects Burn through Causes  Excessive amperage during welding of root.  Excessive root grinding.  Improper welding technique.



Welding Defects

Welding Defects Arc strikes

Spatter

Causes  Electrode straying onto parent metal.  Electrode holder with poor insulation.  Poor contact of earth clamp.

Causes  Excessive arc energy.  Excessive arc length.  Damp electrodes.  Arc blow.



7

Welding Defects

Interpretation of Radiographs Radiographic Details

Mechanical damage

Source of Radiation Screens

150 kV X-Ray

Film Type

Pb 0.125 mm front & back

FFD/SFD

450

Technique

SWSI

Development

Standard

Agfa D7

TWI Training & Examination Services

Chisel Marks

Pitting Corrosion

Radiographic Interpreter

Grinding Marks

Joe Bloggs

Name:

Reference No. 097-200

Date: Material

01/ 01/ 01

Carbon Steel

Welding Details Root Gap

3

Root Face

Process

SMAW

Joint Prep.

Single Vee

Diameter

N/A

1.5

Material Thickness

10



Film Artefacts

 During radiography and film processing images can be formed which are not due to a defect or a change in component thickness.  Such images are referred to as artefacts.


Film Artefacts

Film crimped before exposure


Film Artefacts          

Crimp marks. Dirty intensifying screens. Scratched intensifying screens. Static marks. Reticulation. Solarisation. Chemical or water splashes. Diffraction mottling. Drying marks. Streakiness.


Film Artefacts

Film crimped after exposure


8

Film Artefacts

Dirty intensifying screens


Film Artefacts

Static marks

Film Artefacts

Scratched intensifying screens May appear as either light or dark images often difficult to identify.


Film Artefacts

Reticulation

Release of static electricity due to poor film handling and dry ambient conditions.


Film Artefacts Solarisation  Solarisation is image reversal due to extreme over exposure or exposure to light during development.

Mottled effect caused by extreme temperature change during processing.


Film Artefacts

Water/developer splashes (Before development)



9

Film Artefacts

Fixer/stop-bath splashes (Before development)


Film Artefacts

Film Artefacts

Diffraction mottling Mottled effect sometimes seen in xradiography of large grained materials. Copyright © TWI Ltd

Film Artefacts

Drying marks  Dark marks caused by uneven drying.

Streakiness caused by poor agitation during development



Film Density (a) Weld IQI Type

None

Sensitivity calculation in full

1.4

(b) Parent Material No. of wires or steps visible



2.7 N/ A

N/ A

The film density is less than 2.0. No IQI present. No identification or location markers present. A reshoot is required.



10


Interpretation of Radiographs TWI Training & Examination Services Radiographic Interpreter

Joe Bloggs

Name:

Reference No. 097-201

01/ 01/ 01

Date: Material

Carbon Steel

Welding Details Root Gap

3

Root Face

Process

SMAW

Joint Prep.

Single Vee

Diameter

324 mm

1.5

Material Thickness

8

Radiographic Details

1. 2. 3. 4.

Crater crack, 85 from datum 3 long. Tool mark, 90 from datum. Undercut, 125 from datum, 35 long (intermittent). Wormholes and porosity, 145 from datum, 30 long.



Film Density (a) Weld IQI Type

10ISO16

Sensitivity calculation in full

1.2

(b) Parent Material No. of wires or steps visible

Source of Radiation Screens

180 kV X-Ray

Film Type

Pb 0.125 mm front & back

FFD/SFD

400

Technique

DWSI

Development

Standard

Agfa D7



1.7 2

0.32/ 8 x 100 = 4%

The film density is less than 2.0. The sensitivity is greater than 2%. No identification or location markers present. The IQI is cannot be properly identified. A reshoot is required.



1. Suspected LORF, difficult to interpret due to thickness change, 0-75mm. 2. Porosity, Datum + 120, 45mm long. 3. Lack of penetration, 2 sections 110-130mm and 145168mm. 4. Undercut (cap), intermittent full length, both weld toes.




1. Transverse crack probably caused by Cu pick-up. 2. Scattered pores/wormholes and small slag inclusions. 3. Intermittent minor cap undercut.


11


1. Linear porosity indicating lack of fusion.


1. Tungsten Inclusion.




1. Lack of fusion. 2. Cap undercut. 3. Dense metal inclusions.


1. Linear slag inclusions indicating lack of fusion. 2. Weld spatter.





1. 2. 3. 4. 1. Crack, probably solidification crack.


Lack of root penetration. Burn through. Undercut. Uneven penetration bordering excessive, full length.


12


Part 8: Safety Covering pages 126-138 of your notes



Radiation Safety

 Due to the hazardous nature of ionising radiation it is important to understand the basic principles of radiation safety and have an understanding of the current legislation regarding radiation protection.

Radiation Safety Principles  Justification  Optimisation  Limitation


ALARA


Radiation Safety

Radiation Safety

Units of dose Units of dose

Quality factor

Gray The amount of radiation that will deposit one joule of energy/kg of absorber.


The degree of biological damage caused by a quantity of radiation.


1

Radiation Safety

Radiation Safety Quality factors  X or gamma rays: QF = 1.  Beta particles: QF > 1.  Alpha particles: QF = 20.

Units of dose Sievert: Radiobiological effectiveness, Grays x QF.

Penetrating power  X or gamma: 600mm of steel.  Beta particles: Sheet of paper/layer of skin.  Alpha particles: Less than 1cm of air.



Radiation Safety

Radiation Safety

Safe working Safe working  Controlled area: Any area in which the dose will exceed 3/10th annual dose for employees aged 18 or over.

 Supervised area: Any area in which the dose rate will exceed 1/3rd that of controlled area.

7.5mSvh-1: Maximum dose rate at the barrier.


Shielding


Shielding: Half and Tenth Value layers I

I x2 0

I

I x 10 0

The intensity of radiation is reduced by absorption as is passes through matter. Half value layer

t HVL t TVL

The thickness of any material that will reduce the radiation intensity to one half its initial value.



2

Radiation Safety Calculating safe distances


D1 2  R1  D2 2  R2

D 1 2  R 1

D1: Original distance. D2: Required distance. R1: Original dose rate. R2: Required dose rate.

D2 

 D 2   R 2 2

D1 2  R1 R2




Calculating safe distances: For 20 Ci of Co60

D 1 

2

D2 

Radiation Safety

 R1

D2 

R2

Dose rate at 1m also called output.  Co 60 13 mGy/hr/Ci.  Ir 192 4.8 mGy/hr/Ci.  Yb 169 1.25 mGy/hr/Ci.

1

13mGy / hr / Ci  20Ci 10001 7.5 Sv/hr

Safe distance = 186.2m



Radiation Safety

Radiation Safety Safe distances

Personnel     

 Controlled area: Any area in which the dose is likely to exceed 6mSv/yr or 3/10th annual dose for employees 18+.

Radiation protection advisor (RPA). Radiation protection supervisor (RPS). Classified persons. Trainee. Others.

 7.5mSvh-1: Maximum dose rate at the barrier.



3

Radiation Safety

Controlled Area You are required to follow special procedures to restrict exposure/prevent accidents

Safe distances Supervised area: Any area in which the dose is likely to exceed 1mSv/yr or 1/10th of annual dose for employees 18+.

 During site radiography: Around the radiography position, wherever the dose rate is 7.5Sv/h or more.  Inside an x or gamma radiography compound.  Inside a source store.  Controlled area may be set up following an incident.


Controlled Areas Access is restricted to • Classified workers. • Others working under a written arrangement.      

Local rules. Radiation protection supervisor. Boundary is physically demarcated. Warning notices. Routine radiation monitoring. Personal dosimetry must be used.


Supervised Area There are no special procedures to follow but the area is kept under review by monitoring to pick up any change in conditions Some areas in a radiographic installation:  Where dose rates are higher than background.  Around enclosures where gamma sources are used.



Supervised Area  Boundaries are not physically demarcated.  Warning signs not always legally required but sometimes useful.  No restriction on access.  Routine monitoring is carried out.  Area is described in the local rules.


Local Rules

Written document describing how to:  Carry out the work in accordance with the legislation.  Restrict exposure.  Prevent accidents.


4

Local Rules

Radiation Safety

Read and work in accordance with your employer’s local rules

Monitoring

Local rules will always include:

 Name and contact details of the RPS.  The dose investigation level.  Description of any controlled or supervised areas.  Written arrangements for non classified people working in a controlled area.  Summary of the contingency plan.


Survey Meters Survey meters produce a reading of the current dose rate, usually in mSv/h or mSv/h. Three types are used in industrial radiography: 1. Geiger counters. 2. Ionisation chambers. 3. Scintillation counters.


Survey Meters  Ionisation Chamber

AMMETER

When the gas is ionised a current can flow through the chamber. The magnitude of the current is related to the intensity of ionising radiation.


      

Ionisation chamber. Geiger muller tube. Scintillation counter. Film badge. Thermo-luminescent dosimeter (TLD). Quartz fibre electroscope. Audible monitors (personal monitor).


Survey Meters  For the detection of X or gamma radiation geiger counters are usually used.  Geiger counters are effectively high voltage ionisation chambers.  They are designed to produce pulses of current when exposed to radiation.  The number of pulses produced can be related to the radiation dose rate.  Geiger counters are more compact and more durable than standard ionisation chambers and have a wider measurement range.


Survey Meters  Scintillation counters are extremely sensitive to low levels of radiation.  They are useful for checking for contamination.  A scintillation counter uses a phosphor which flashes in the light spectrum when exposed to ionising radiation.  Flashes of light are detected by a photomultiplier tube.  Different phosphors are used for different applications: eg sodium iodide for X and gamma ray detection or zinc sulphide for alpha particles.


5

Radiation Dose Monitoring  Quartz fibre electroscopes, film badges, thermo-luminescent dosimeters and some types of personal monitor are all devices for measuring total radiation dose over a period of time.  TLDs use a lithium fluoride (LiF) phospor.  When exposed to ionising radiation LiF stores energy which is later released as flashes of light when the phosphor is heated.


6

2. NDT20 Course Notes

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