Small, shielded cadmium telluride (CdTe) semiconductor gamma-ray detectors have been used for in situ radiological characterization of underground high-level radioactive waste tanks. Remote measurements up to 700 R/h have been made in gamma radiation fields. Spectral data have been used to generate qualitative and quantitative radionuclide profiles of high-level radioactive waste tanks. Two electronic spectral enhancement techniques (pulse risetime discrimination and pulse risetime compensation) have been used in order to measure trace isotopes in the presence of large amounts of {sup 137}Cs. Spectral resolution better than 1.5% FWHM for the {sup 137}Cs 662 keV photopeak has been obtained. 4 refs., 7 figs

Addleman, R. S.

Blewett, G. R.

Keele, B. D.

McClellan, C. S.

Subrahmanyam, V. B.

Troyer, G. L.

UNT Digital Library

WHC-SA-1196-FPApplication of a _.=o_,_,_. -'" CdTe Gamma-Ray _-_ -_,- Spectrom,ete_r _.to Remo_teCh a racte rizat ionof High-LevelRadioactiveWaste Tanks• Prepared for the U.S. Department of EnergyOffice of Environmental Restoration and Waste Managementt@ Westin_HanfordCompanyRi:r,,_,_,J,w_. ._o._anfcrc ,_;erau_ns and En§_neenng Contractor for t_e,. S D_artment _f Energy unuer Contract DE-ACQ6-87RLIO£30Approved for Public ReieaseLEGAL DISCLAIMERmrh,s -door: ,_as areoare(_ as af_ account of work soonsore_ 3yan age,,:,cyof the Un,tea Stales Government. Ne_Lher:neUn_:ec States Gaver'-:men: "!or any agency ,,_ereoi, nor any of:ne,r emo_ayees, qor an').'ai :he,r con',rac,ars, suocontractars Ior:'eremoiofees, ma_es any warrar_y exDressor,mDl!ea,Qr ass,Jmes any :ecja, ,ao_i_ty or resuons,mhty for '.heacc.racy :'.Dmoieteness, _r a_y tmrg oar'.y's use or [he resultsof s..c" .ce of a_y ntcrma!_on aaaaratus aroduc:, or orecassc:sc:osed or,euresen_s:,_at,ts Jsewou_d not,ntr_nge9r'vate,y owned r_gr_:s Referencenere{n "o any s_3ec:ficcammerc.:a 9rO£;UCt. 3rocess or servtce oy _rane name,tracemar_, maruiacturer or otr'erw_se, aces no; necessar_;ysoPs:tote cr redly :s er_oorsement, recommenoattan, or'avcr'qg 3y me dr,,;ec States Government or any agency:_ereo_ or ',s cantrac',ors or su_3contrac*,ors The wews anct_o_ cns a_ authors exaressec _,ere_nde qo_ necessar,ty stateor "e{'ec • :r'cse o+ :he Umted Slates Governmem or anyager_cy :'e reoiT'_;s 'ostr: _as aden -earad_cea !tom '.Me aesr avat!anle col3y::_' ':'eC q t_e .;" te_ :_lale:5 Of _ri'1_'_iC_WHC-SA- -i 196DE92 002714Application of a CdTe- Gamma-Ray Spectrometer, to Remote Characteri_tionof High-Level RadioactiveWaste TanksB. D. KeeleR. S. AddlemanG. R. BlewettC. S. McClellanV. B. Sub_ma.nyamG. L.TroyerDate PulalishedOctober 1991To Be Presented at To Be Published in1:991 IEEE NuclearScience Symposium IEEE TransactionsandMedical ImagingConference on Nuclear ScienceSanta Fe, New MexicoNovember5 - 9, 1991. Prepared forthe U.S. Department of EnergyOffice of Environmental Restorationand Waste ManagementIIRichland,Washington99352HanfordOpera_onsandEngineeringConlractorfortheU.S.DepartmentofEnergyunderContractDE-AC06-87RL10_OmCin a _, nm/aly4mm Ill.mm in irict to amy ¢==pTngm om_ mis ImlD_.Approved for Public Release MASTEtApplication of a CdTe G3mma-Ray Spectrometer to RemoteCharacterization of High-Level Radioactive Waste Tanks*R. S. AddlemanG. R. BlewettB. D. KeeleC. S. McClellanV. B. SubrahmanyamG. L. TroyerQWestinghouseHanford Company, Richland, WA 99352,Abstract Measurementsof h_gher energy gamma rays,aOCo,_S2Eu,and ISCEuhave been successful.Small, shielded cadmium telluride Measurement access is l_mited to(CdTe) semiconductor gamma-ray detectors penetratingdrywells (typically 3 in. inhave been used for _n situ radiological diameter) located on the interior andcharacterizationof undergroundh}gh-level exterior of the HLWTs. Probes containingradioactive waste tanks. Remote the detector,shielding,preamplifier,andmeasurementsup to 700 R/h have been made an auxiliary cooling system have beenin gamma radiation fields. Spectral data constructed to fit within the drywells.have been used to generate qualitativeand Gamma radiation fields within the HLWTs- quantitative radionuclide profiles of can reach 700 R/h.high-level radioactivewaste tanks. Small, 2 x 2 x 2 mm, planarTwo electronic spectral enhancement configuration cadmium telluride (CdTe)techniques (pulse risetime discrimination detectors manufactured by Radiationand pulse risetime compensation)have been Monitoring Devices, Inc. were used forused in order to measure trace isotopes in this application. CdTe's high atomicthe presence of large amounts of 137Cs. number providessignificantstoppingpowerSpectral resolution better than 1..5%FWHM and good peak-to-total ratio for smallfor the 1_Cs 662 kev photopeak has been detectors.obtained. The traditional problem of chargetrapping with CdTe detectors has beenI. INTRODUCTION partiallyovercome by incorporationof twospectralenhancementtechniques. With theA large portionof the nations high- pulse risetime discrimination (PRD), level radioactive waste is stored at the technique, a large fraction of events areHanford Site in large underground storage rejected based on risetime in a mannertanks. Characterization of high-level similar to references I and 2. Pulsewaste tank (HLWT) contents is one of the risetime compensation (PRC) is an event" first steps toward the cleanup of the amplitude correction technique b_Red ontanks and the environmentalrestorationof the risetime of the event. Referencesthe Hanford Site. and 4 describe the use of this techniqueRadionuclide content and with germanium detectors.distributionin HLWTs have been determinedby in situ gamma-ray spectroscopy. Thepredominategamma-rayemitter in the HLWTsis 137Cs. The large abundance of 137Csmakes measurementsof other isotopes withphoton energies below 662 keV impossible.*Performed under DOE Contract DE-AC06-87 RL10930.II. DETECTOR AND ELECTRONICCONFIGURATIONA. Pulse Risetime Discrimination Fr eg n of Pulse AcceotanceThe PRD was used as a method to eelectronically enhance photopeak qresolution. At the preamplifier output u' ethe risetime of each event (pulse) noccurring within the detector is cdetermined. Pulses that lie within a y- narrow range of risetimesare acceptedforcounting,and the remainder of the pulses ] "_".......-"-".............................are rejected. Approximately half of the Risetime• events are rejected.Me electronicconfigurationfor thePRD system is shown in Fig. I. The pulse Fig. 2 CdTe pulse risetime spectrumrisetime analysis system consists of asingledelay-line amplifier,a pulse-shapeanalyzer, and a time-to-amplitude Fig. 3 displays a gamma-ray energyconverter with an integral single-channel spectrumwith a 2 x 2 x 2 mm CdTe detectoranalyzer (SCA). The pulse-shape analyzer taken with and without PRD. It was foundfunctions in the same manner as two that pulses of any selected risetimeconstant fraction discriminators. This produced quality gamma-ray energy spectraprovides a risetime spectrum which is as illustratedin Fig. 4. Slower risingindependentof the gamma-rayenerg2/region events generate photopeaks which areutilized, shifted to lower channels than fasterCdTe rising events. The risetime spectrum isOetecrGr reproducible at different shaping timesI[Prea_,_,_erl thus; the effect is due to charge trappingI ' , and not ballisticdeficit. During slower,' Ioe'avUneiAm_,,f,errising events, charge is resident withini_te__.,egra_,ng [ the crystal for longer periods of time;', _,_,_e_ [p_Js_s_aoG the holes and electrons have a greaterl i........A_a_yze_! probability to recombine or becomeLoe,ayLJ_) s'_'_lIS,o_ trapped. This leads to lower amounts ofTACw'm I charge collection, which describes theI,me£ralSCA photopeakshif s bserved. The unenhanceds,g_,,_,i _,_,_,sc_I _c spectrum shown in Fig. 3 is a compositeAoc#_ [ADC_2I gamma-rayenergy spectrumwith events fromADC-..-,o_,oD,_,t_Co_.,_. al I ri set i mesSCA-Single Channel AnalyzerT,a_C-Tir_ to Aml31itt_e Conv_rteroFig. 1 PRD block electronicsFig. 2 represents a typicalrisetimedistribution of preamplifier outputpulses. Pulses within the highlightedrange of risetimes were accepted forcounting, the remainder were rejected., By selecting a specific risetime,spectralresolutioncan be improvedat the B. Pulse Risetime Compensationexpense of detector efficiency. Using a• PRD, a resolution of !.5% full width at The PRC technique adjusts thehalf maximum (FWHM) for 137Cswas obtained, amplitude of the pulse by an amountA gated integrating amplifier with proportionalto the square of the risetimea shaping time constant of 0.25 /_secwas of the event. This is performed w_th anused because of the high radiation fields analog computer that was developed toencountered. The gated integrator was correct for ballistic deficits inused to overcome a ballistic deficit germanium detectors and to correct forresulting from short shaping times. The charge trapping in neutron damagedPRD effectively performs pulse pile up germanium detectors. The computer isrejection, commercially available from EG&G ORTEC,and is called a germanium resolutionenhancer. This technique counts everyevent while maintaining satisfactoryresolution.The electronicconfigurationfor the improvewith bias voltage in Fig. 7. ThePRC system is diagrammed in Fig. 5. This detector has been routinely operated atsystem consists of a gated integrator 200 V for extended periods of time.which provides the spectral signal, aspectroscopy amplifier from which therisetime is derived, and the resolutionenhancer.FDetector •' I _U t_j Preamlolitier 1 • _'l n i " ;' ',l F_.._, /_.S'CJ" fTt, t'18, _" l ' JArnloll fler Amplifier .... .---_--.---_7_.'_.._....2....--_.WSigr_/ Bt_olar RWwN_OutDut Outgut....... liOV _lK) V .... 120V "_" 200 VJ Ge Resalution i_I Ennencer tFig. 6 The effect of bias voltage onthe risetimeFig. 5 PRC block diagramIn theory, the germanium resolution r i""""'" _ 66_/k__enhancer shouldcorrect for both ballistic edeficit and charge trapping within the QLJCdTe crystal. In practice, spectral equality is improved by using the gated nintegrator to overcome the ballistic cYdeficit and by using the resolutionenhancer to overcome charge trapping. . , , , --Fig. 3 contains gammA-ray energy spectra Energytaken of a 137Csand =%0 source using thePRC system. Using the PRC system, .....60v --_2o v --_.oovphotopeak resolution of 2.5% FWHM at662 keV was achieved while counting every Fig. 7 The effect of voltage on anevent, unenhanced 137Csgamma-ray energyspectrumC. Detector Temperature and Bias- VoItage DependencyIII. RESULTS AND DISCUSSIONThe maximummanufacturerrecommended° operatingtemperaturefor the detectorwas Spectral resolutions of 1.5% and48 °C. It was noted that operation above 2.5% FWHM @ 662 keV have been obtained32 °C would producegain shifts as well as using PRD and PRC. This resolution isbroadening of the pulse risetime spectra, better than sodium iodide scintillatorsBecause the internal temperaturesof some but not as good as germanium or siliconHLWTs were over 80 °C, the detector was semiconductordetectors. Because of thecooled with circulating ice water, high atomic number of CdTe, the peak-to-Varying the detector bias changed total ratio is a factor of 10 better thanthe shape of the risetime distributionas an equivalent sized germanium detector.shown in Fig. 6. When the detector was The electronic enhancementbiased below 100 V, the risetime techpiques, PRD, and PRC have beendistributionwas unstable and subject to successful in comper.satingfor problemslarge drifts in both peak position and associatedwith chargemobility and chargeshape. Photopeak quality can be seen to trapping. The PRD selects events withuniformcharge trapping based on selecting VII. REFERENCESevents with similar risetimes. The PRDresults in the best resolution at the [I] L.T. Jones and P. B. Woollam,expenseof rejectingmany events. "ThePRC "Resolution Improvement in CdTecompensates for charge trapping by Gamma Detectors Using Pulse-Shapeadjusting the signal based on the square Discrimination,"NuclearInstrumentsof the risetime. The PRC results in good and Methods, vol. 124, pp. 591-595,energy resolution while counting all !975.events. [2] L.T. Jones, "The Use of CadmiumPerformance and stability of CdTe Telluride Gamma Spectrometers indetectors were observed to significantly Monitoring Activity Deposited inimproveat higher bias voltages. Nuclear Power Stations," Revue DeCdTe detectors, with electronic Physique Appliquee, Vol. 12, No. 2,enhancements, have proven useful for pp. 379-384, Feb. 1977.application in gamma-ray spectroscopy [3] F.S. Goulding and D. A. Landis," within confinedspaces with high radiation "Ballistic Deficit Correction infields. The specific advantages of CdTe Semiconductor Detectorare as follows- Spectrometers,_ IEEE TransactionsonNuclearScience, Vol. 35, No. I, pp.• Small size 119-124, Feb. 1988.• Room temperatureoperation [4] M.L. Simpson, T.W. Raudorf,• High Z material providing good T.J. Paulus, and R. C. Trammel,stopping power and moderate "Charge Trapping Correction in Geresolution Spectrometers,"IEEE Transactionson• Low cost Nuclear Science, Vol. 36, No. I,• High count-rate operation. Feb. 1989.IV. FUTURE WORKFuture improvements will includelocatingthe origin of the timing signalsat the preamplifierlevel, customizingthepreamplifierto match the characteristicsof CdTe more closely, and performing PRCdigitally for improved resolution.Implementingthese changes will probablyimprovethe resolutionavailablefrom CdTesystems.It appears that if a detector canbe made such that higher bias voltages canbe successfully applied, no enhancement. electronicswill be necessary.VI. ACKNOWLEDGEMENTSWe would like to thankW. K. Hensleyof Pacific Northwest Laboratory for hisadvice that led to the presentwork.WHC-1196-SA-FPDISTRIBUTIONNumber of copiesONSITE5 U.S. Department of Enerqy-RichlandOperations OfficeR.F. Chri_tensen (5) E6-3321 WestinqhouseHanford CompanyR. S. Addleman (2) T6-50G. R. Blewett (2) T6-31B. D. Keele (8) T6-31C. S. McClellan (2) S0-61V. B. Subrahmanyam (2) T6-.-31G. L. Troyer (2) T6-50InformationReleaseAdministration(2) L8-07Document Processing andDistribution L8-15Distr-1Ir

Application of a CdTe gamma-ray spectrometer to remote characterization of high-level radioactive waste tanks

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Application of a CdTe gamma-ray spectrometer to remote characterization of high-level radioactive waste tanks

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