Gamma-Ray Detectors

1 3 gamma -RayDetectorsHastingsASmith,Jr., , , , ,eitherdirectly(aswithaproportionalcount erorasolid-statesemiconductordetector)or indirectly(aswithascintillationdetector) , ,wewillpresentsomegeneralinformationonty pesof gamma -raydetectorsthatareusedinnondestru ctiveassay(NDA) , , ( )Inmostdesignstheouterelectrodeisthecyli ndricalwallofthegaspressurevessel,andthe inner(positive) (especiallyofionizationchambers) . 4 l%escintillationandsolid-statedetectorsa remuchmoredesirableforobtainingthe~spect roscopicdetailneededintheenergyrangetypi calofuraniumandplutoniumradiation(approx imately100-1000keV).Gascountersaredescri bedinmoredetailinChapter13,sincetheyarem orewidelyusedforneutrondetection. - - - * ~teCtO~Thesensitivevolumeofascintillatio ndetectorisaluminescentmaterial(asolid,l iquid,orgas)thatisviewedbyadevicethatdet ectsthegamma-ray-inducedlightemissions[u suallyaphotomultipliertube(PMT)].Thescin tillationmaterialmaybeorganicorinorganic ; ,plastics, (thestandardagainstwhichotherscintillato rsarecompared).

2 Somecommoninor-ganicscintillationmateria lsaresodiumiodide(NaI),cesiumiodide(CSI) ,zincsulfide(ZnS),andlithiumiodide(LiI). Themostcommonscintillationdetectorsareso lid, ,bismuthgermanate(BiqGesOl2),commonlyref erredtoasBGO,hasbecomepopularinapplicati onswhereitshighgammacountingefficiencyan d/oritslowerneutronsensitivityoutweighco nsiderationsofenergyresolution(Refs. 3and4). ,2, ,ionized(excited)atomsinthescintillatorm aterial relax , ,the (calledactivators) [NaI(Tl)].Thescintillationlightisemitted isotropically;sothescintillatoristypical lysur-roundedwithreflectivematerial(such asMgO)tominimizethelossoflightandtheniso pticallycoupledtothephotocathodeofaPMT.( )Scintillationpho-tonsincidentonthephoto cathodeliberateelectronsthroughthephotoe lectriceffect, ,theycollidewithelectrodesinthetube(know nasdynodes) ,causingalargemultiplication(byafactorof 104ormore)oftheelectronfluxfromitsinitia lvalueatthe gamma -RayDetectors47 SOLID-STATECRYSTALSIGNAL,-1*.

3 /.:,;.~.:~nDEPLETED(SENSITIVE)REGION~c ::: ~::: ::R$,..,,:,,.:.pHIGH:VOLTAGE+~ (seeChapter4).(a)(b)(c) :(a)open-endedcylindricalortruecoaxial,( b)closed-endedcylindrical,and(c) , (theso-calledtruecoaxial)orclosed-endedc rystals[ (a-b) ,Jr.,andMarciaLucastheelectricfieldforch argecollectionisprimarilyradial, ,theradialeiectricfieldgeometrymakesthec oaxial(especiallytheopen-endedcoaxial) [forexample, (c)]. ,withthesmallthicknessesoptimumforlow-en ergymeasurements(forexampleintheL-x-rayr egionforspecialnuclearmaterial).Planarde tectorsusuallyachievethebestenergyresolu tion,becauseoftheirlowcapacitance;theyar epreferredfordetailedspectroscopy, , ,inenvironmentswhereneutronlevelsarehigh (suchasaccelerators,reactors,orin-strume ntswithintenseneutronsources), ,radiationdamageeffectscanbeofconcerninN DAapplicationswherelargeamountsofnuclear materialarecontinuouslymeasuredwithhigh- resolution, gamma -rayspectroscopyequipmen t forexample, ,reducestheamplitudesofsomefull-energypu lses, ,theresolutionisdegraded, ( ).]

4 Ithasbeengenerallyobservedthatsignifican tperformancedegradationbeginswithaneutro nfluenceofapproximately109n/cm2,anddetec torsbecomeunusableatafluenceofapproximat ely1010n/cm2( ).However, ~iirming(annealing)thedetectorcrystal( ). ,2, ,thereverse-bias-diodeconfigurationofage rmaniumsolid-statedetectorresultsinveryl owcurrentsinthedetector(usuallyinthepico -tonanoampererange).Thisleakagecurrentca nbefurtherreducedfromitsroom-temperature valuebycryogeniccoolingofsolid-statemedi um,typicallytoliquidnitrogentemperature( 77K).Thiscoolingreducesthenatural,therma llygeneratedelectricalnoiseinthecrystalb utconstitutesthemaindisadvantageofsuchde tectors:thedetectorpackagemustincludecap acityforcooling,andthisusuallyinvolvesad ewarforcontainingtheliquidcoolant. Inrecentyears,attemptshavebeenmade- gamma -RayDetectors491I,~n= ,! <!,,!t *. ~.:; ;.#~ viI_ . ~\-*<#nqt&@$#$:, keV;!,d-t,~~..,. :t10,/i4006008001000I 0= ~ ,:.;>;!%.*.* .; 1I1[.. ~-d~;&..J~~~ III qj$s$i,- !]

5 ~;&/$102004006008001000 -2 XIO*ln/cm21$ (GAMMAENERGY)Thedeteriorationofahigh-res olutionsolid-statedetectorgammaspec-trum withincreasingneutronjhence(4). ( ) ,Jr.,andMarciaLucastocoolthedetectormate rialelectronically( ),buttheseeffortsarestillintheexperiment alstages, [Si(Li)].Theloweratomicnumberofsiliconco mparedtogermaniumreducesthephotoelectric efficiencybyafactorofabout50(seeChapter2 ),butthistypeofdetectorhasbeenwidelyused inthemeasurementofx-rayspectrainthe1-to5 0-keVenergyrangeandfindssomeapplicationi nx-rayfluorescence(XRF)measurements(seeC hapter10).Thelowphotoelectricefficiencyo fsiliconabove50keVisanadvantagewhenmeasu ringlow-energyxraysandgammarays, ,aswellasmanyotherapplica-tionsofgamma-r ayspectroscopy,itwouldbeadvantageoustoha vehigh-resolutiondetectorsoperatingatroo mtemperature, ,Hg12,andGaAshasbeenextensivelyresearche d( ).Theirhigheraverageatomicnumbers, ,thesedetectormaterialshaveenjoyedlimite dap-plicationtoNDAproblemstodate, ,thesedetectorsmaybecomemoreattractiveas convenient, ~-RayEnergyAtomice-hPair(6)aResolutionat 122keV~MaterialNumbers(eV)(keV)Ge(77K) (300K)48, (300K)80, (300K)31, (300K)c11, Thisquantitydeterminesthenumberofchargec arriersproducedinaninteraction.

6 ( )bRepresentativeresolutiondata, Whilenotasemiconductormaterial, , (MCA), (spectrum)ofthedetectedoutputpulses, , (a). :(1)thequantumuncertaintiesintheenergies ofthetransi-tions(theso-calledHeisenberg Uncertainty),and(2) , ,the ideal , , (b). (b) ,afterleavingthesample,willbescatteredby externalmaterialsbeforeenteringthedetect or,andthiseffectcanshowupinthefinalenerg yspectrum(seebelow).Whenthegammarayenter sthedetectionmedium,ittransferspartorall ofitsenergytoanatomicelectron, ,inaseriesofcollisions, (seeTable3-1).Aphotoelectricinteractiont ransfersalloftheincidentphoton senergytoaphotoelectron; ,Jr., (~~~j~EOPULSESFROMSINGLEPHOTONINTERACTIO NSINTHEDETECTOREc1t1 -RayEnergyE;(a)(b)(c)Anidealizationofthe photonspectrum(a)producedbyPeenuclei,(b) emergingjiomamaterialsample,and(c) - . ]ypartoftheincidentphoton senergytoanionizedelectron; ;however, (c).Themaximumenergythatcanbedepositedin thedetectionmediumfromaComptonscattering eventcomesfromaneventwherethephotonissca tteredby180.)

7 TheCompton-generateddetectorpulsesarethe reforedistributedbelowthismaximumenergy[ (c)]andconstituteasourceof background (c) ( ). )A3n MONOENERGETICGAMMA-RAYFLUXGEcBEoDETECTOR PULSEAMPLITUDE(y-RAYENERGY) ,Jr., , ( ). ,distributedsmoothlyuptoamaximumenergyEC ( ), (EC) head-on collisionbetweenthephotonandtheelectron, wheretheelectronmovesforwardandthegamma- rayscattersbackwardthrough180 ( ). Comptonl%lley. Foramonoenergeticsource,pulsesinthisregi onarisefromeithermultipleComptonscatteri ngeventsorfromfull-energyinteractionsbyp hotonsthathaveundergonesmall-anglescatte ring(ineitherthesourcematerialsorinterve ningmaterials) , -120 ,amonoenergeticsourcewillgiverisetomanys catteredgammarayswhoseenergiesarenearthi sminimumvalue( ). ,eventsinthisregionarefromhigh-energygam maraysandcosmic-raymuonsinthenaturalback groundandfrompulse-pileupeventsifthecoun trateishighenough(seeChapter4).Inamoreco mplexspectrum, ,verynearthe zero-pulse-height-amplitude region,arisestypicallyfromlow-amplitudee lectronicnoiseinthedetectionsystemthatis processedlikelow-amplitudedetectorpulses (seeChapter4), ,containingmanydifferentphotonenergies,t heCompton-edgeandbackscatterpeakfeatures tendto ~ullwidthofthe(full-energy)photopeakatHa lfits~aximumheight(FWHM).

8 IfastandardGaussianshapeisassumedforthep hotopeak,theFWHM isgivenbyFWHM=2um(3-1) (smallFWHM) , ,suchasnoise,pulsepileup,improperpole-ze rosettings, , (Tl) ~ , , ,Jr.,andMarciaLucas ~3in, (Tl)$.[ !.L~2iii10qp.: I Averageenergy(ineV)requiredtoproduceonee lectron-ionpairinthedetectingmedium.~The ratioE/dforE=300keV(Equation3-2).cThequa ntityo(n)/n,or(l/n)l/2,withouttheincorpo rationoftheFanofactor. Thestatisticalportionoftheenergyresoluti on,AESt~t, , ~geom=A/(47rr2)(3-8) - - gamma -RayDetectors59whereAisthecross-sec tionalareaofthedetectorandristhesource-t o-detectordistance( ). $ciency& (suchasthedetectorhousing,specialabsorbe rs,etc.) (itshouldbe<<1) [ (flpx),],thesampleefficiencyis1 exp[ (ppx)s]T 1 Esample=(PPX)S=x.(3-lo) ~~ , exp( ppx)(3-11)wherepisthephotoelectricmassat tenuationcoefficient, ,s~ ,e~~tcanusuallybeapproximatedbyapowerlaw oftheformqntuaE~b.(3-12)Anotherimportant termisrelativeefficiency,whichhastwocomo tations:lRelativetoNalItiscommonpractice tospecifytheefficiencyofagermaniumdetect orat1332keV(60Co) :e,etto~al(Ge)=100etot(Ge,1332keV)/ctot( NaI,1332keV).]

9 (3-13) ,Jr.,andMarciaLucasThetheoreticalvalueof eto~(NaI,1332keV) ,forexample,a30%(relative) YA169~b-169~m\\n154~u_154Gd\l39K(n,y)40K v 2C(n,y)13C\ CURVEFITTEDTOEXPERIMENTALDATAJ1___EXPERI MENTALCURVE,CORRECTEDFORABSORPTIONBETWEE NSOURCEANDSENSITIVEDETECTORVOLUME1,o-f3~ 04 PHOTONENERGY(keV) (Li)detector( ). (C~bSP);thedecreaseineficiencyathighener gyillustratesthedecreasedinteractionrate inthedetectorcrystalforhigher-energygamm arays(E~ ,Jr.,andMarciaLucas10101\\\\+153&j-153E A 169yb-16gT~n154E _154G(j+53Cr(n)y)54Cr~12C(njy)13C\A N(n,y)15N CURVEFITTEDTOEXPERIMENTALDATA$$-2II-LLjJ J~II111111 III11111/-3_-4 -5 III111111 III111111I11111P----EXPERIMENTALCURVE,CO RRECTEDFORABSORPTIONBETWEENSOURCEANDSENS ITIVEDETECTORVOLUMEIk10 102103104 PHOTONENERGY(keV) ,exceptforapointsource83mmfromthefaceofa 38-cm3truecoaxialGe(Li)detector(adaptedf romRef15). , , , (orx-ray) ,thephotonenergiesofmajorinterestintheND Aofnuclearmaterialrangefrombelowthex-ray region(85-100keV) (seeChapter9),plutoniumisotopicmeasureme ntsinthe400-to1000-keVrange(seeChapter8) ,andoccasionalmeasure-mentsof238 Udaughteractivityinthe600-to1000-keVrang e.)

10 (Themajorgamma-raysignaturesfornuclearma terialarelistedinTable1-2.)Aswasillustra tedinthedis-cussionabove,detectorsthatar ethickintheaxialdimensionaremoreefficien tforthehigh-energyapplications,andforlow -energygamma-andx-raymeasurements, ,suchascostandportabilitymaydictatetheus eoflessexpensiveandmoreportableNaI(scint illation) Detectors , , , ,RadiationDetectionandMeasurement(JohnWi ley&Sons,Inc.,NewYork,1979). ,AppliedGamma-RaySpectrometry(PergamonPr ess,NewYork,1975). , , , BismuthGermanateScintillators:Applicatio nsinNuclearSafeguardsandHealthPhysics, NuclearInstrumentsandMethodsA242,480(198 6). , ,andJ, , ImprovementsintheEnergyResolutionandHigh -Count-RatePerformanceofBismuthGerminate , NuclearInstrumentsandMethodsA242,369(198 6). ,TheTheoryandPracticeofScintillationCoun ting(PergamonPress,Oxford,1964). , , , EffectsProducedbyFastNeu-tronBombardment ofGe(Li) gamma -RayDetectors; NuclearInstrumentsandMethods62,173(1968) . ,Jr., , , , , DeteriorationofLargeGe(Li)DiodesCausedby FastNeutrons, NuclearInstrumentsandMethods98,481(1972) .