1、%5,7,6+ 67$1$5%6 5HFRPPHQGDWLRQ IRUDWD RQVKLHOGLQJIURPLRQLLQJUDGLDWLRQ 3DUW6KLHOGLQJ IURP JDPPD UDGLDWLRQ8 other terms commonly encounteredare thekilocurie(kCi)equalling103curies,millicurie(mCi)equalling103curie,and themicrocurie(Ci)equalling106curie.Theactivity,andhence theradiationoutput of the so
2、urcedecays with timeinanexponentialmanner.Aconvenient measureof therateofdecay is givenby thehalf-lifeofa source, whichrepresents the time takenfor the sourceactivity tofallby afactorof two.Toenablerapidassessment of theactivity ofa source tobedeterminedgivenits half-lifeandoriginalactivity,agraphis
3、 given(Figure1)of thefractionalreductioninsourceactivityas afunctionof thenumberofhalf-lives passed sinceits originalactivity was known.Thus forexample,considering24Na withahalf-lifeofhours,ifit is assumed that theoriginal sourceactivity was Qcuries,and theactivity after84hours is required,proceedas
4、 follows:Numberofhalf-lives passed=FromFigure1 thiscorresponds toafractionalreductionof0.02.Hence the sourceactivity is0.02 Qcuries.Conversely,if theexistingactivity is Pcuries, then theactivity84hours previously wasP/0.02 curies.The values ofhalf-lives aregivenin thedata sheets for theradionuclides
5、 considered.Aknowledgeof thehalf-lifeis necessaryifitisdesired tocalculate theactivityofa sourcefor whichagiven shield wasoriginallydesigned,havingknowledgeof theexposurerateat the timeofcalculation.3 Calculation of exposure ratefrom a point sourceIn this section,considerationis only given to theexp
6、osuredue to theprimary beam.Theeffects of scatterareconsideredinAppendixB.Fora sourceofgammaradiation, theexposurerateat agivenpositiondepends on thefollowing:i) theradionuclideconcerned,andits specificgamma-ray constant;ii) the sourceactivity;iii) thedistancebetween the sourceand thepositionofinter
7、est (theexposurerate willbeinverselyproportional to thedistance squared);iv) the typeand thicknessof shieldmaterialplacedbetween thepositionand the source.a)Unshielded source.For thepurposesof this recommendation, the specificgamma-rayconstant foragiven sourceis takenas beingequal to thegamma-ray ex
8、posurerateinroentgens perhourat adistanceof1mfroma sourceofactivity onecurie.NOTE Otherdefinitions exist,andcare shouldbe takenthat thedatain this recommendationare usedinconjunction withspecificgamma-ray constants in these terms.8415-5.6=BS 4094-1:19661 The British Standards Institution 2015BS 4094
9、-1:19662BSI 09-1999Figure 1T he relation between remaining activity and half-livesBS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 09-1999 3Thus fora sourceofactivity Qcuries, theexposurerateat dmfrom the sourcein theabsenceofany shieldingmaterialbetween the sourceandpositionof
10、measurement maybedeterminedas follows:Exposureratefromonecurieat onemetredistance=+ R/hwhere + is the specificgammaray constant for that source.Therefore,exposureratefromQcuries at 1m= +QR/handhence,exposureratefromQcuries at dm=the termarisingfrom theinverse squarelaw.Values of the specificgamma-ra
11、y constant + aregivenin thedata sheets; + is plottedas afunctionofphotonenergy inFigure8.b)Shielded source transmissionfactor.Thepresenceofa shieldbetween the sourceandadefinedposition willreduce theexposureratebelow the unshielded valuedue toabsorptionin the shield.Thus for the sourceconsideredina)
12、andagiven shield, theexposurerateat adistancedmfrom the sourceis givenby:Exposurerate=whereTis the transmissionfactor, whichdepends on thematerialand thickness of the shield,and thetypeof source.Values of the transmissionfactoraregivenin thedata sheets in theformofgraphs showing transmissionas afunc
13、tionof thickness for various shieldingmaterials.Thedatagivenrefer tobroadbeams ofradiation(seeAppendix A,SubsectionsA.2and A.3).4 Data sheetsData sheets areprovidedfor thefollowing sources:Theinformationnecessary tocalculate theradiation transmitted through various shieldingmaterials is givenon thed
14、ata sheets foreachof the sources.Theinformationon the sheets includes:Theradionuclideanddecay schemeRadioactivehalf-lifeSpecificgamma-ray constant(+)Broadbeam transmissioncurves for various materials.Examplesi)Suppose that theexposureratedue toa192Ir sourceat thepoint tobe shieldedis2.5R/hand that i
15、t is required toreduce this to0.75mR/h.The transmissionfactor,T,is givenby0.75 103=T 2.5FromFigure 2h(i), therequired thickness of shieldingis5.7cmPb.ii)Considera60Co sourceof200 curies, shieldedby lead,andassume that becauseof thelayout of thelaboratoryin whichit is being used, thenearest distanceo
16、fapproachofpersonnelis twofeet.It is required todetermine the shielding toreduce theexposurerateat this point to2.5mR/h.Fromdata sheet No. 2, the specificgamma-ray constant (+)for60Co=1.32 R/Ci.hat1m.24Na50Co85Kr124Sb137Cs170Tm182Ta192Ir198Au226Ra+decay products232Th+decay products+Qd2-R/h1d2-+QTd2-
17、 R/hi.e. T0.751032.5- 0.0003= =BS 4094-1:19663 The British Standards Institution 2015BS 4094-1:19664BSI 09-1999Hencefora sourceof200 curies,theexposurerateat2 feet(0.61m)from the source= 7.1 102R/hSince therequiredexposurerateis2.5 103R/h,a transmissionfactorof =3.52106is required.FromFigure 2b(i) t
18、herequired thickness of shieldingis22.5cmPb.iii)If,however,it is required todetermine the shield thickness togiveanexposurerateofXmR/hat theshield surface,as fora transport container, theprocedureis morecomplicatedbecausedistance,andhence thegeometry factor and the transmissionfactor,are variables.I
19、t is recommended that thefollowingprocedurebeadopted:Let Xbe therequiredexposurerateat the shield surface;whereTtis the transmissionfactorfora shieldof thickness t(cm)anddis thedistanceinmetres from thesource totheouter surfaceof theshield.ThusEvaluate using transmissiondatafora series of values ofd
20、 untilaquotient of therequired valueis found;hencethenecessary thickness ofshieldingis obtained.It shouldbenotedthat,incertaincircumstances,dandtmay beequal.200 1.320.612- R/h2.5 1037.1 102-1d2-then X+QTt103d2- mR/h,=Ttd2-BS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 09-1999
21、 5Section 2. Data sheets and associated broad-beamtransmission graphsNOTE Transmissiongraphs inthis sectionarefor thefollowingdensities ofbrick,concreteandbarytes concrete,Forbrick,concreteandbarytes concreteofotherdensities,it is sufficiently accurate toapply acorrectionfactorequal to theratioofthe
22、densities.Theeffectiveness of the shieldincreases withincreasingdensity of the shieldmaterial.Brick 1.6 g/cm2Concrete 2.35g/cm2Barytes concrete 3.2 g/cm2BS 4094-1:19665 The British Standards Institution 2015BS 4094-1:19666BSI 09-1999DATA SHEET No. 1SODIUM-24(24Na)Half-life:15.0 hours.Specificgamma-r
23、ay constant(+):1.84R/Ci.hat1m.Transmissionfactors:SeeFigure 2a.Decay scheme:Radiation:BetaGammaEnergy MeV: 1.39100%1.37100%2.75100%BS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 09-1999 7Figure 2a(i) Transmission of gamma rays from24Na through barriers of various materialsBS
24、4094-1:19667 The British Standards Institution 2015BS 4094-1:19668BSI 09-1999Figure 2a(ii) Transmission of gamma rays from24Na through barriers of various materialsBS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 09-1999 9DATA SHEET No. 2COBALT-60(60Co)Half-life:5.26 years Spec
25、ificgamma-rayconstant (+):1.32 R/Ci.hat1mTransmissionfactors:SeeFigure2b.Decay scheme:Radiation:BetaGammaEnergy MeV: 0.31100%1.17100%1.48approx.0.01% 1.33100%BS 4094-1:19669 The British Standards Institution 2015BS 4094-1:196610BSI 09-1999Figure 2b(i) Transmission of gamma rays from50Co through barr
26、iers of various materialsBS 4094-1:196610 The British Standards Institution 2015BS 4094-1:1966BSI09-1999 11Figure2b(ii)Transmissionofgammarays 60Cothroughbarriers ofvarious materialsBS 4094-1:196611 The British Standards Institution 2015BS 4094-1:196612BSI 09-1999DATA SHEET No. 3KRYPTON-85(85Kr)Half
27、-life:10.6 years Decay scheme:Radiation:BetaGammaEnergy MeV: 0.15 0.7% 0.510.7%0.6799.3%Specificgamma-ray constant (+):Theoretical(not takingbremsstrahlungintoaccount): 0.0012 R/Ci.hat1metre.85Krluminous signs:0.020 R/Ci.hat1m.Transmissionfactors:SeeFigure 2c.BS 4094-1:196612 The British Standards I
28、nstitution 2015BS 4094-1:1966BSI 09-1999 13Figure 2c Transmission of gamma rays plus bremsstrahlung from luminoussigns filled with85Kr through lead barriersBS 4094-1:196613 The British Standards Institution 2015BS 4094-1:196614BSI 09-1999DATA SHEET No. 4ANTIMONY-124(124Sb)Half-life:60 days Decay sch
29、eme:Radiation:BetaGammaEnergy MeV: 0.2211% 0.6099%0.6151% 0.65 8%0.95 6% 0.7214%1.59 7% 0.97 3%1.66 2%1.05 2%2.3123%1.331%1.375%1.45 2%1.7050%2.09 6%Specificgamma-ray constant (+):0.98R/Ci.hat1m.Transmissionfactors:SeeFigure 2d.BS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 0
30、9-1999 15Figure 2d(i) Transmission of gamma rays from124Sb through barriers of various materialsBS 4094-1:196615 The British Standards Institution 2015BS 4094-1:196616BSI 09-1999Figure 2d(ii) Transmission of gamma rays from124Sb through barriersof various materialsBS 4094-1:196616 The British Standa
31、rds Institution 2015BS 4094-1:1966BSI 09-1999 17DATA SHEET No. 5CAESIUM-137(137Cs)Half-life: 30.0 yearsDecay scheme:Radiation:BetaGammaI.C.Energy MeV: 0.5195% 0.66286%9%1.175%Specificgamma-ray constant (+): 33 R/Ci.hat1m.Transmissionfactors:SeeFigure 2e.BS 4094-1:196617 The British Standards Institu
32、tion 20150.33 R/Ci.h at 1 m.BS 4094-1:196618BSI 09-1999Figure 2e(i) Transmission of gamma rays from137Cs through barriers of various materialsBS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 09-1999 19Figure 2e(ii) Transmission of gamma rays from137Cs through barriers of variou
33、s materialsBS 4094-1:196619 The British Standards Institution 2015BS 4094-1:196620BSI 09-1999DATA SHEET No. 6THULIUM-170(170Tm)Half-life:127 daysDecay scheme:Radiation:BetaGammaI.C.Energy MeV: 0.8822% 0.0843%19%0.9778% 0.0524%X-raysE.C.0.15% 0.0073%Specificgamma-ray constant (+):0.0025R/Ci.hat1m.for
34、A.E.R.E.sources,includingX-ray contribution.0.0035R/Ci.hat1m.for solutions.High valuearises frombremsstrahlungcontribution.Transmissionfactors:SeeFigure 2f.BS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 09-1999 21NOTE Sourcegeometry influences bremsstrahlungproductionandhence
35、 the shapeof the transmissioncurves; this is exemplifiedin thecurves foraluminium.Figure 2fT ransmission of gamma rays from170Tm through barriers of various materialsBS 4094-1:196621 The British Standards Institution 2015BS 4094-1:196622BSI 09-1999DATA SHEET No. 7TANTALUM-182(182Ta)Half-life:115days
36、Decay scheme:Radiation:BetaGammaI.C.EnergyMeV: 0.1838% 0.066 3% 7%0.25 5% 0.06831% 3%0.33 2% 0.085 2%12%0.3620% 0.10013%43%0.4423% 0.114 2%4%0.48 4% 0.1564%0.51 8% 0.179 5% 2%0.198 3%1%0.22212%1%0.229 7%1%0.264 7%1%1.00 3%1.12 33%1.19 15%1.22 28%1.23 11%1.252%1.292%0.059(X-rays)Specificgamma-ray con
37、stant (+):0.68R/Ci.hat1m,includingX-ray contribution.Transmissionfactors:SeeFigure 2g.BS 4094-1:1966 The British Standards Institution 2015BS 4094-1:1966BSI 09-1999 23Figure 2gT ransmission of gamma rays from182Ta through barriers of various materialsBS 4094-1:196623 The British Standards Institutio
38、n 2015BS 4094-1:196624BSI 09-1999DATA SHEET No. 8IRIDIUM-192(192Ir)Half-life: 74.4daysDecay scheme:Radiation:BetaGammaI.C.Energy MeV: 0.24 6% 0.29630% 2.5%0.5440% 0.30829% 2.5%0.6750% 0.31683%5.7%0.46853%1.5%E.C.4% 0.588 6%0.60512%0.613 7%Specificgamma-ray constant (+): 0.48R/Ci.hat1m.Transmissionfactors:SeeFigure 2h.BS 4094-1:196624 The British Standards Institution 2015
