The major difficulty encountered in the determination of beta-ray dose in field conditions is generally the presence of a relatively high gamma-ray component. Conventional dosimetry instruments use a shield on the detector to estimate the gamma-ray component in comparison with the beta-ray component. More accurate dosimetry information can be obtained from the measured beta spectrum itself. At Los Alamos, a detector and discriminator circuit suitable for use in a portable spectrometer have been developed. This instrument will discriminate between gammas and betas in a mixed field. The portable package includes a 256-channel MCA which can be programmed to give a variety of outputs, including a spectral display, and may be programmed to read dose directly

Brake, R. J.

Eisen, Y.

Erkkila, B. H.

Unruh, W. P.

Wolf, M. A.

UNT Digital Library

A major purpose OTme I txr IIII-cal Information Center is to providethe broadest dissemination possi-ble of information contained inDOE’s Research and DevelopmentReports to business, industry, theacademic community, and federal,state and local governments.Although a small portion of thisreport is not reproducible, it isbeing made available to expeditethe availability of information on theresearch discussed herein. ‘LA-UR -84-3389I os Alfimoe Nwond L~MfWOV 10opemtd by ma UnwersiW 01 Cdfofnlc fof Iha United Stareo Depemnent of Energy under conlrwl W.740$ENG-3S.TITLE:AUTHOR(S):SUBMITTED TOILA-UR--84-3389rlEf!50!)2370A BETA-GAMMADISCRIMINATORCIRCUITB. H. Erkkila,IT-1M. A. Wolf,IT-1Y. Eisen,HSE-1W. P. Unruh,IT-1R. J. Brake,HSE-1.,~. ,,,:,1984IEEENuclearScienceSymposium,October31 -November2, Orlando,FloridaDISCLAIMER‘rhlnraprt wan fwepnrcd nsunnwmmiofwork fipIJfIMWdhyUfIISCnCy of llMUniled StnletOtwernmanl, Ncllhcr Ihc IJnkd Wnten (kwcrnmwrl nor UIIy uaency thcrw)f, nor wry of hairemployees, makcn #ny wurrnmy, caprcnnor implied, or uwrumcnany legal Iltibillty or roqmns!-blllly for the uccurncy, ~mmplelenew,or usdulrwm or nny Mormnlion, uppmtun, product, orprwxsn cIIscI(w6, or reprcncntsthal lIn UIWwould nut Infringe prlvMy owned rlghtn. Refer.ence herein ‘ ~nny specific ~wmmercud rrroducl, pocenn, or nervlu hy trade name, Irmdam-rh,mnnufncturcl. m olherwkx dmo nol necewwily wrnlhule or Imply III endureemenl, rocommend-lion, or rworlng hy the (Jnlled Slulen (hwemmanl m tii,y agenuy keof, The views●nd opinlmrn or nuthorn cRprcsnedhcreln do nol neuensnrily stnle m rclleci thoee of theUnited Stnlen ( hwernmont wr my ~uency thcrcof,By accotrtanceof trw WIICIO,MmUubhmor ?Wognllas Ihd Iha US (lWornmcnl mlmne a nOnWCluSIW,royally.hw llc~ee roBublmhor raprorlucaWIOPUMIIIW fnrm o! IhIS conlrleullon, or 10 Muw othem 10 do W. M US aovornmwu mmaeoe.lhg l.o~ nlnmor Nwonal L~bO?MOfVreuuasm IFIoIlM publmhw IdMWY llw@ MIcI. as wwh dwmod undof Uw Uepraa d !h@ U S DerMrlmont of CnsfgyLos NationalLaboratoryA BETA-GAMMA DISCRIMINATOR CIRCUITB. H. Erkkila, M. A. Wolf, Y. Eisen,W. P. Unruh and R. J. BrakeLos Alamos National LaboratoryLos Alamos, New Mexico 87545The majordeterminationconditions isAbstractdifficulty encountered in theof beta-ray dose in fieldgenerally the presence of arelatively high ‘gamma-ray compoient. Conven-tional dosimetry instruments use a shield onthe detector to estimate the gamma-raycomponent in comparison with the beta-raycomponent. More accurate dosimetry informa-tion can be obtained from the measured betaspectrum itself. At Los Alamos, a detectorand discriminator circuit suitable for use ina portable spectrometer:havs been developed.This instrument will discriminate betweengammas and betas in a mixed fietld. Theportable package includes a 256-channel MCA[1] which can be programmed to give a varietyof outputs, including a spectral display, andmay be programmed to read dose directly.introductionAn instrument has been developed at LosAlamon which is intended to provide a portablebeta spectrometer and dosimeter. The majorproblem of discriminating ~etween gamma andbeta events hds been overcome. A combinationof special detector and electronics has beendeveloped which is capable of discriminatingbetas from gammas of modorate energy. Betasinternct continuously along a path as theyenter a scintillator, while gamma interactionstake place at statistically distributed pointswithin the sclntillator. This differenceprovides a means of discriminating volumetri-cally against gammaiawhile at the ~ame timeidentifying betas as thoj enter the datector.General Descriptio~—. —The heart of the instrument is thespecisl detector that ‘S shown in Figure 1.It consists of a plastic phoswich detectorwith a thin aluminized mylar window. Thefront scintillator is 3 mm thick and has adecay time of ’250 no. The rear scintillatoris 2 cm thick and hat) a decay timo of <10 ns.The front scinti!.laterthickness lU chosen toErovlde a7ood compromise between reliableeta detact.on al~d1)W gamr,~acrass section, Athickness of 3 mm is sufficient to extract30-40 keV from the entering betas. The rearscintillator shsuld be thick enough to totallystop the most energetic betas of interest, andit must have an efficiency which matches thatof the slow scintillator. Discriminationagainst gammas results from the fact thatentering betas deposit same energy in thefront scintillator while very few gammas willinteract with it because of its small volume.Compton electrons produced in the fast scin-tillator, wh!.ch then enter theSci-tillator,thin sloware generally of somewhat lowerenergy than the energy deposited by the enter-ing betas and can ke partially discriminatedagainst.The electronics required to accomplishthis discrimination is shown in block diagramform in Figure 2. The average waveshapesobtained from the PMT are shown schematicallyin Figure 3. Output from the PMT is extractedthrough two paths. An integrating preampli-fier Is driven by the llth dynode and a fastlinear amplifier is driven by the anode. Theintegrated signal sums the total energy dep.s-ited in both scintillators, and these pulsesprovide the spectroscopic iniormatlon. Thefast amplifier signal is fed to a discrimina-tor to detect the pulse start timing and tothe input of a fast gated integrator. Ordi-nfirily,the threshold of this first discrim-inator is set to reject thermionic noise fromthe PMT. The pulse start signal from thediscriminator is delayed ’100 ns and thentriggers a 300 ns wide gate signal to thegated integrator. The output of this integra-tor feeds a second discriminator which ser’~esto detect the presence of a siqnal from theslow scintillator. If the PMT signal has thetail shown in Figure 3c, there will be signi-ficant outpu: from the fast qated integratorand therefora from the second discriminator.The presence of this output serves to indicatethat the event was due to an event in the slowscintillator, most likely an li~cident betaparticle. The slow integrated signal obtainedfrom the llth dynode is collected by a multi-channel analyzer (MCA) with a gated ADC. Thisgate is obtained from the beta Jetection cir-cuit?ust described and? when run in thecoinc dence mode~ S11OWS the MCA to recordpulses only from beta events.QhservationsThe initial development was done on asystem made up oi t41Mmodules and a commer-. .cially available MCA. Typical results areshown in Figure 4. The ungated spectrum shownin Figure 4a was taken in a mixed field withabout 5 gammas to 1 beta. Figure 4b shows thespectrum obtained in this mixed field with thebeta discrimination applied. A gated spectrumof just the beta source is shown in Figure 4c.As can be seen from the total counts recordedin each case, the discrimination ●gainstgammas is about 50:1, in good agreement withthe ratio of 67:1 expected for the scintilla-tor thicknesses used.After it was established that this methodof discrimination was feasible, work wasstarted on design of the electronics suitablefor a portable instrument. The MCA USed was avariation of the un?.tsdeveloped at Los Alan?os[1,2] for a variety of applications. Thefront end electronics have been designed andpartially developed. We anticipate thisinstrument will work well i,lgamma fields ofenecgy up to approximately 1.5 MeV.Fast plastic scintillators typicallyexhibit a slow afterglow of a few percentrwith a time constant of about 2 VS. Thicemission interfere with the discriminationagainst gammas of energy appreciably higherthan 1-2 MeV. Since there is wide applicationfor an instrument capable of discriminatingagainst higher energy gammas, it is ofinterest to decrease this effect significantlycr to eliminate it completely. The severaldifferent fast plastics we have examined havesignificantly different levels of afterqlow.This suggests that a material might be pro-duced with much lower delayed emission. Apromising material has been found in whjch theafterglow has been reduced by special doping.Work is progressing on construction of adetector package which will allow this circuitto perform the discrimination agains..gammasof several MeV energy.Another approach being pursued is toseparate the two scintillator signals opti-cally by coupling them separately to two PMTs,thus removing any energy dependence in th~discrimination. This scheme has the advantageof using simpler electronit:s? as no fastamplifiers or delays are needed. The thinacintillator may have a fast decay time~ sothat light pipe losses can be partially com-pensated by using a high efficiency plast!c.Preliminary measurements with a detector ofthis type are vary encouraging.SummaryA practical method has been demonstratedto obtain a usable beta spectrum in a mixedbeta-gamma field. This is useful in fieldwork where a pure beta field 1s seldomencountered. The method of discriminationdescribed can be implemented in a field--portable instrument capable of both measuringthe spectrum and calculating the skin dose, aswell as the dose at any tissue depth in themixed field. Such an instrument is now infinal development.References[1] C. J. Umbarger, M. A. Wolf, and F.Trujillo, “Violinist: Sophisticated PuField Monitor,” IEEE Trans. on NuclearScience, Vol. NS 31, No. 1, Feb., 1984,Pg. 659.[2] M. A. Wolf, H. C. Staley, “simple MCASu portsEVariety Applications,”su mitted to the ?;84 IEEE NuclearScience Symposium.TYPE A-SPECTROMETER /DOSIMETEFI0.3mmNE-1 15 OR EC-444——.HAMAMATSU R329PMT3mg/cm 2ALUMINIZEDMYLAR \Smm NE-111 OR BC-420PLASTICFigure 1Figure 2ooy OnlyFast Scintillator+ ‘“e --’1III I i I ~J._J100 200 500Time (nsec.)Low Energy pSlow ScWlllator100 200 300 400 500Time (neoc.)IIHigh Energy pBoth Scintlllatorst--- ‘at” +\-—————————.~-500Time (neec.)Figure 3400030002000666500Ei111191i11i9j91R1ijk1R 119i*111191L“%I b 1049001000Lc 900001000 I:% j b%n I,, ,,ls. ,,l*nnnl**l, la, ,.1.“o 200 4C3 600 800CHANNEL NUMBERFigure 4

Beta-gamma discriminator circuit

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Beta-gamma discriminator circuit

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