CHAPTER 3: R ADIOACTIVITY
Band of Stability Predicting type of radioactivity Kinetics of radiation Applications of radioactivity
3.1. BAND OF STABILITY 3.1.1. All nuclei with !"e th#n $3 %"!t!n& #"e un&t#'le . Elements with 83 protons or fewer may also have nstable nclei! depending on the ratio of netrons to protons. "f nmber of netrons is plotted against nmber of protons for stable nclei! a characteristic graph is obtained #$"%. 3.1.&.
'he plot prodces a band of stable nclei called the '#n( !) &t#'ilit*. A section of the band is shown in more detail at element 3(! which has si) stable nclei. Elements which lie otside the band ndergo "#(i!#cti+e (ec#*. 'his prodces a new ncles which may or may not be radioactive itself. 'he process contines ntil a stable ncles is formed. 'his can be seen more clearly by nderstanding natre of three main types of radiation.
3.,.
AND
RADIATION
3.*.1. N#tu"e !) "#(i#ti!n+ 'his is the loss by an nstable ncles of two protons and two netrons as a single ,particle. An ,particle is therefore a helim ncles+
-ote that the ratio of netrons+protons changes becase they are removed in a different ratio from that which e)ists in the parent atom. owever! the 'hallim ncles prodced by the decay is still nstable and it mst ndergo the second type of radiation # & before a stable ncles reslts. 3.*.*. N#tu"e !) "#(i#ti!n+ /hen ,radiation occrs! the ratio of netrons+protons is redced becase a netron changes into a proton. At the same time an electron is prodced! and this is lost from the ncles as a! so, called! ,particle+
3.*.3. N#tu"e !) -"#(i#ti!n+ 0ring , and ,decay! e)cess energy may be released as high freency electromagnetic radiation 2nown as ,radiation.
3.3. PREDICTIN THE TYPE OF RADIATION 3.3.1. /% t! eleent $,! nclei which have too high a ratio of netrons+protons ndergo ,radiation! bt not ,radiation. Above 8*! elements with too high a ratio can ndergo either , or ,radiation. -ote also! that elements with too low a ratio of netrons+protons ndergo a different type of decay in which a proton is converted into a netron! and a %!&it"!n is released+
owever! sch nclei are not natrally occring! bt they may be prodced by nclear reactions.
3.0. S/ARY OF THE PROPERTIES OF 3.(.1. P"!%e"tie& !)
-,
-,
#n(
-RADIATION
-"#(i#ti!n+
i2 N#tu"e+ $ast moving helim nclei! ths positively charged. ii2 Beh#+i!u" in #n elect"ic )iel( + 0eflected towards the negative plate. iii2 Beh#+i!u" in # #netic )iel( + 0eflected according to $lemings left hand rle #$"%. 3.4.&+
-ote that the direction of flow of the conventional crrent.
,particles 5 the direction of flow of
i+2 I!ni&in %!we"+ they pass throgh.
,particles have a powerfl ionising effect on any gases
+2 Penet"#tin %!we"+ paper. 3.(.*. P"!%e"tie& !)
,radiation is absorbed by 6cm of air or by a sheet of
-"#(i#ti!n+
i2 N#tu"e+ $ast moving electrons! ths negatively charged. ii2 Beh#+i!u" in #n elect"ic )iel( + 0eflected towards the positive plate! and deflected to a greater e)tent than ,particles owing to the low mass of an electron. iii2 Beh#+i!u" in # #netic )iel( + 0eflected according to $lemings left hand rle! and ths in the opposite direction to ,radiation! as well as to a greater e)tent. i+2 I!ni&in %!we"+ ,particles are less ionising than a,pariticles as predictable from their lower mass and lower 2inetic energy. +2 Penet"#tin %!we"+ ,radiation can travel a few metres throgh air! and throgh thin sheets of metal. 'he denser the metal! the thinner the sheet that can be penetrated. 3.(.3. P"!%e"tie& !)
-"#(i#ti!n+
i2 N#tu"e+ igh freency electromagnetic radiation. ii2 Beh#+i!u" in #n elect"ic )iel( + 7naffected. iii2 Beh#+i!u" in # #netic )iel( + 7naffected. i+2 I!ni&in %!we"+ /ea2ly ionising. +2 Penet"#tin %!we"+ ,radiation can pass throgh several 2ilometers of air throgh p to 14cm of lead.
3.4. 5INETICS OF R ADIOACTIVE DECAY 3.4.1. R#(i!#cti+e eleent& (ec#* according to first order 2inetics #section 8.1.&+ the rate is proportional to the nmber of radioactive atoms present! and the half,life is constant #section .1.*. and table .1.&.
"n this conte)t! h#l)-li)e is the time ta2en for half the srcinal nmber of radioactive atoms to disintegrate. 0ring this period! the intensity of radiation will obviosly fall to half its srcinal vale. 3.4.*. An e6u#ti!n+ "f yo wish to remember an eation describing the rate of radioactive decay! remember this one+
.. 9o may have the misfortne of encontering sch a mathematical manoevre. opeflly! yor e)aminers will not reire yo to ta2e part in one.
3.7. APPLICATIONS OF R ADIOACTIVITY 3.:.1. C#"'!n (#tin+ 'he concentration of radioactive 1(carbon dio)ide in the atmosphere is assmed to have been constant throghot history #abot 1 molecle in 1;!;;;&. 0ring their lifetime! living organisms absorb radioactive carbon! either dring photosynthesis #plants& or indirectly via feeding #animals& on other living organisms. "t is therefore assmed that throghot history the proportion of radioactive to non,radioactive carbon in living organisms has been constant.
/hen a living organism dies! it stops absorbing radioactive carbon and the radioactive carbon decays with a half,life of 446; years. By measring the ratio of radioactive to non,radioactive carbon in material derived from living organisms! it is therefore possible to estimate its age since death. 3.:.*. T"#ce"& #n( l#'ellin + 'he fate of a molecle in a living organism can be traced by labelling the molecle with a radioactive isotope. "n this method! one atom in each of the molecles to be traced is replaced with a radioactive isotope. $or e)ample! a particlar carbon atom in each molecle of a sample of glcose can be replaced by 1(carbon. #"n fact! replacement is not 1;;<.& "f the glcose is fed to an organism! the fate of the glcose 1(carbon atom can be traced by detecting and locating the ,radiation. 'his can provide information abot the types of molecle prodced from the glcose! and the location of those molecles within the organism and its cells. owever! labelling is not e)clsive to biochemistry and medicine. $or e)ample! by replacing the o)ygen atoms in an ester with 18o)ygen! it is possible to determine which bond is bro2en dring ester hydrolysis #section **.3.:.i.&.
3.8. 9/ESTIONS 1& Accont for the different behavior of
-,
-,
and ,radiation in
i& an electric field! ii& a magnetic field. *& =omment on the following statements+ i& "sotopes are radioactive atoms of an element. ii& alf,life is half the time ta2en for a sample of a radioactive element to decay totally. iii& 'here is more similarity between ,radiation and light! than there is between ,radiation and ,radiation. 3& $ill in the missing data #indicated by estion mar2s& in the following schemes. 9o will need a periodic table to identify the named elements.
.. /ill the final ncles at the end of each chain be stable> (& ow valid are the assmptions on which carbon dating is based> #'he 1( carbon isotope is prodced in the atmosphere by the bombardment of nitrogen by cosmic rays.& 4& ow wold yo se hydrolysis>
18
o)ygen to determine which bond is bro2en dring ester