Gamma-Ray Spectroscopy Using A NaI(Tl) Detector

1 DEPARTMENT OF PHYSICSINTEGRATED LABORATORYG amma-Ray Spectroscopy UsingA NaI(Tl) DetectorObjectives:Becomefamiliarwithsom eof thebasictechniquesusedformeasuring (gamma)rays witha sodiumiodide(NaI) : ,ExperimetalPhysics, (OxfordUniversity Press:NewYork,1988).Chapters11 & 12. ,Experiments in ModernPhysics, (AcademicPress:NewYork,1966).Chapter5. :Experiments in NuclearScience, publishedby EG&GORTEC,wastheprimarysourceformuch of thematerialdiscussedin :Binandpower supplyNaI(Tl)crystalandphototube assemblyAmpli er(Ortec570,485,or equivalent)Highvoltagepower supply(Ortec456or equivalent)MaestroMultichannelanalyzer(M CA)emulatorprogram,andcomputerwithMCAcar dRadioactive sources:137Cs,60Co,22 NaImportant:Duringthisexperiment, always remember thatyouareworkingwithradioactive until youhave discussedhow to ,always usethetweezersprovidedandensurethatthera diationsafety signsprovidedarepostedin therelevant IS NOTINUSE, : Electronicblock diagramfor -ray Spectroscopy systemwithNaI(Tl).

2 Many radioactive isotopes decay by ,thedecay is oftennotto thegroundstateof thedaughterisotope, butto to lower energystatesby emitting rays untilthegroundstateis rays canbe detected,andtheirenergiesmeasured,usings everaltypes of thislab,theuseof NaI(Tl) scintillatoras a -ray detectorwillbe (Tl)detectoris notsensitive to the particles,sincethesearenotlikelyto penetratethedetector' unknownsfora given sourcethatwe wouldlike to theenergiesof the rays fromthesource,whiletheotheris thenumber of rays thatleave thesourceperunitof time(activity). In thisexperiment, thestudent willbecomefamiliarwiththemeasurement of thesepropertiesusingNaI(Tl).Experiment:T hisexperiment willbe brokenupintoa seriesof exercises,each of which shouldbe completedby thestudent.

3 Beforebeginningtheexercises,thestudent shouldbecomefamiliarwiththecomputer' thisexperiment, theMCAwillbe runas a pulse-height analyzer(PHA),whereeach channel(orbin)correspondsto a smallrangeof event in thedetectorproducesa voltageproportionalto theenergyof the -ray which causedtheevent. Theevent is thenaddedto thechannelcorrespondingtoitsvoltage(ener gy)range,andtheresultingspectrumis thus a plotof intensity (number of counts)versusvoltage(energy).ExerciseI - EnergyCalibration:In thisexercise, calibratetheNaI(Tl) Setuptheelectronicsas shownin Figure1. Therearetwo parametersthatultimatelydeterminetheover allgainof thesystem:thehighvoltagethatis furnishedto thephototube andthegainof thelinearampli thephotomultipliertube is quitedependent generalruleformostphototubes is thata 10%changeof thehighvoltagewillchangethegainby a factorof Setthehighvoltageto Settheampliferforpositive inputandbipolaroutputfortheOrtec485; Placethe137 Cssource 10 cmin front of theNaI(Tl) Adjustthecoarseand negaincontrolsof thelinearampli erso approximatelychannel280, theillustrationsshownin Figure2, thegainof thesystemhasbeensetso that1 MeVfallsat aboutchannel420to linear,2 MeVwouldthereforefallat channel840-850(forthe1024setup).

4 Donotchangethehighvoltageor ampli Accumulatethe137 Csspectrumlongenoughto , a leon 'sbestto save lesin a more secure locationthantheC: Replacethe137 Cssourceby samplespectrumis showninFigure3. Againnotethechannelnumbersforthephotopea ks, a Make a plotof thephotopeakenergiesfor137 Csand60 Coversuschannelnumber andgenerateacalibrationcurve withwhich youcanconvertchannelnumber to - Analysesof the137 Csand60 CoGamma-Ray Spectra:Thepurposeof thisexerciseis to explainsomeof thefeatures,otherthanthephotopeaks,thata reusuallypresent in a pulse-height a purekinematiccollisionbetweena -ray photonandwhatmight be termeda freeelectronin theNaI(Tl) ,theincident -ray gives uponlypartof itsenergytoFigure2: Typical137 Csspectrummeasuredusinga NaI(Tl).

5 Typical60 Cospectrummeasuredusinga NaI(Tl) given to therecoilelectron(andtheintensity of thelight ash)dependson whetherthecollisonis head-onor a head-oncollision,the -ray thescattered -ray canbe determinedby termsof thescattered -ray canbe writtenapproximatelyas:E 0 =E 1 + 2jE j(1 cos )(1)whereE 0is theenergyof thescattered -ray in MeV,E is theenergyof theincident -ray in MeV,jE jis thedimensionlessmagnitudeof theincident -ray and is thescatteringanglefor verifythatit is = 180 dueto a head-oncollisionin which 0isscattereddirectlyback, becomes:E 0 =E 1 + 4E (2)Asanexample,we willcalculateE 0foranincident -ray energyof 1 MeV:E 0 =1M eV1 + 4E = 0:20M eV(3)Theenergyof therecoilelectron,Ec, forthiscollisionwouldbe truesinceEc=E E 0(4)4 Figure4: Variousevents in thevicinity of a typicalsource-crystaldetector-shieldcon theComptonedge,which is themaximumenergythatcanbe impartedto anelectronby theComptoninteraction,canbe -rays make Comptoninteractionsin demonstratesthevariousevents thatcantake placein a typicalsource-NaI(Tl) -rays fromtheseinteractions(E 0) make photoelectricinteractionsin theNaI(Tl) thebackscatteredpeack canbe foundby Calculatetheenergyof -rays fortheedgeandyourcalibrationchart,does themeasuredvalueagreewiththecalculation?

6 2. Solve forthebackscattered -rays -rays - EnergyResolution:Theresolutionof a spectrometeris a measureof itsabilityto resolve two peaksthatarefairlyclosetogetherin energy. Figure2 showsthe spectrumthatwas thephotopeakis foundby solvingthefollowingequation:R= EEx 100%(5)whereRis theresolution(inpercent), Eis thefullwidthof thepeakat halfof themaximumcount level(FWHM)measuredin number of channelsandEis thechannelnumber at thecentroidof Figure2, thephotopeakis in channel280andtheFWHM= , theresolutioniscalculatedto be Calculatetheresolutionof : Gammadecay TheActivity of a -Emitter:Anotherunknownassociatedwiththe gammasourceis theactivity of thesource,which is usuallymeasuredin Curies(Ci),where1 Ci= 1010disintigrationsper thesourcesthatareusedinnuclearlaboratory experiments have activitiesof theorderof microcuries( Ci).

7 1. fromthefaceof Accumulatea spectrumfora xedlengthof Determinethesumunderthephotopeak, Cs, referringto themanualforthemultichannelanalyzerif Erasethespectrum,remove thesourceandaccumulatethebackgroundforth esamelengthof timetanddetermine Usetheformulabelow to caclulatetheactivity of = Cs reft 1 absf(6)wheretis thelive timein seconds, absis theabsolutepeake ciencyforthisdetector,fora particular -ray energy,fis thedecay fractionof theunknownactivity which is a fractionof thetotaldisintegrationsin which themeasuredgammais emitted(seeFigure1),anddsis thesource-to-detectordistancein ciency( abs) have beendeterminedusinga a verycommontechniquein -ray Spectroscopy , sinceit is oftendi cultto obtainaccuratee ciencymeasurements fora madeusingGEANT4,a cylinderof NaI, diameter, height was surroundedby a shellof aluminum(Figure5).

8 Five million rays werethen redrandomlyin alldirectionsfroma point ciencywas thendeterminedto be abs=Counts in thepeak pCounts in thepeakTotalemitted rays:1(7)Valuesdetermined(aspercentages) forallthee cienciesnecessaryforthislabaregivenin ,includea shortdiscussionof possibleproblemswithusingsimulationto determinee basedon the Poissondistribution,for which the uncertainty inNis : Simulatedgeometryof theNaIdetector,shownwithpartof : E cienciesfordi erent -ray energies(1 beingthehighest), - TheMassAbsorptionCoe cient:In thisexercise,thestudent willmeasureexperimentallythemassabsorpti oncoe cient in leadfor662keV outthat -rays interactin matterprimarilyby photoelectric,Compton,or cient canbe easilymeasuredwitha -ray thisexercise,we willmeasurethenumber of -rays thatareremovedfromthephotopeakbythephoto electricor Comptoninteractionsthatoccurin a leadabsorber 'slaw, thedecreasein intensity of radiationas it passesthroughanabsorber is given by:I=Ioexp( )(8)whereIois theintensity beforetheadditionof theabsorber,Iis theintensity aftertheadditionof theabsorber and is thedensity thicknessin g/cm2.

9 Thedensity thicknessis theproductof thedensity (ing/cm3) andthethickness(incm).Thehalf-valuelayer (HVL)is de nedas thedensity thicknessof theabsorbingmaterialthatreducestheorigin alintensityIoby 50%. , it is a relativelysimplematterto show that:HVL=0:693 (9)In thisexperiment, we willmeasure in -rays : Decay schemefor60Co1. (Tl) ( Cs- ref) to be at ( Cs- ref).2. ErasetheMCAandinserta pieceof leadfromtheabsorber of live timeas in Step1 above. Determine( Cs- ref).3. ErasetheMCAandinsertanotherpieceof ( Cs- ref).Repeatwithadditionalthicknessesof leaduntil thecount-sumis< Ona semilogscale,plottheintensityIversusabso rber thicknessin mg/cm2, whereI= ( Cs- ref)/live andcalculate How does yourvaluecomparewiththeacceptedvalueof Repeattheabove withtheacceptedvalueof SumPeakAnalysis:Figure3 showsthetwo showsthedecay thetime,thedecay occursby -particleemissionto decay to thegroundstatealways occursby -ray emissionto (a -ray),followedalmostinstantaneouslyby -ray emissionto events arein coincidenceandhave anangularcorrelationthatdeviatesfromanis otropicdistributionby only16%.

10 For thepurposesof thisexercise,we canassumethateach of these -rays otherwords,if 1goes in a particulardirection, 2cangoin any of the4 steradiansthatit a certainprobability thatit willgo in thesamedirectionas 1. If thisoccurswithintheresolvingtimeof thedetector, 1and 2willbe summedandhencea sumpeakwillshow upin of counts underthe 1photopeakis given by: 1= 1f1tA(10)whereAis theactivity of thesampleandtis a similarcalculation,thesumunderthephotope akcorrespondingto 2is: 2= 2f2tA(11) ,thenumber of counts in thesumpeak, sis given by: s= 1 2f1f2tA[W(0 )](12)whereW(0 ) is a termthataccounts thecaseof60Co, :8 Figure7: Decay schemefor22Na(inset)andtypicalspectrumme asuredusingNaI(Tl) Detector . s 60Co= 1 2tA(13)sinceW(0 ) = ,andf1andf2arealso = (seeFigure1).