Low Temperature Performance of a Large Area Avalan

1LowTemperaturePerformanceofaLargeAreaAvalanchePhotodiodeV.N.Solovov*,F.Neves,V.Chepel,M.I.Lopes,R.F.MarquesandA.J.P.L.PolicarpoLIP-CoimbraandDepartmentofPhysicsoftheUniversityofCoimbra,3004-516Coimbra,PortugalAbstractALargeAreaAvalanchePhotodiode(LAAPD)wasstudied,aimingtoaccessitsperformanceaslightdetectoratlowtemperatures,downto–80ºC.Theexcessnoisefactor,F,wasmeasuredandfoundtobeapproximatelyindependentofthetemperature.AlineardependenceofFontheAPDgainwithaslopeof0.00239±0.00008wasobservedforgains100.Thedetectionoflowintensitylightpulses,producingonlyafewprimaryelectron-holepairsinthephotodiode,isreported.PACS:85.60.Gz,85.60.Dw,70.70.+mKeywords:photodetectors,excessnoisefactor,avalanchephotodiode,LAAPD,*Correpondingauthor:[e-mail]lanchePhotodiodes(LAAPD)areofgreatinterestforhighefficiencydetectionoflowintensitylightpulsesfromscintillators.Inspiteofsomedrawbacks,theycancompetesuccessfullywithphotomultipliertubes(PMT)inmanyapplicationsthatrequireverycompactdetectorpackaging(inPositronEmissionTomography,forinstance),lowsensitivitytomagneticfields(asinsomespaceapplicationsandacceleratorexperiments)orlowintrinsicradioactivityasrequiredforthelowbackgroundexperiments(e.g.,searchofdarkmatter).VerygoodtimeandenergyresolutionshavebeenachievedwithLAAPDswithvariousscintillators,namelyinorganiccrystals[1,2]andliquidxenon[3].Operationatlowtemperaturecanbenotonlyanecessity,asinthecaseofliquidxenon[3],butalsoafavorableoption.Infact,tooperatetheLAAPDatlowtemperaturehastwoadvantageouseffects:1)thedarkcurrentnoisedecreasesdramatically;2)thevoltagerequiredtoachieveacertaingainbecomeslower.Inourpreviouswork[4],botheffectswereinvestigatedasafunctionofthetemperaturebetween25ºCand-100ºC.InadetectorwithaLAAPDreadout,thenoiseandthefluctuationsoftheavalanchegaincontributetotheenergyresolutionofthesystem.Thelaterisusuallyexpressedintermsoftheexcessnoisefactor,F,[5].Previousmeasurementsoftheenergyresolutioninliquidxenonexcitedwithalpha-particles[3],indicatedavalueofFsignificantlylargerthanthevalueusuallyreportedforroomtemperature.Inthispaper,wereportonmeasurementsoftheexcessnoisefactorasafunctionofthegainatlowtemperatures(downto–80ºC).Theobservationofverylowintensitylightpulsesthatproduceonlyafewprimaryelectron-holepairsinthephotodiodeisalsopresented.32.ExperimentalSet-UpInourmeasurementsweusedawindowlessLAAPD,5mmindiameter,fromAdvancedPhotonix,Inc[6].Thisphotodiode,manufacturedusingbeveled-edgetechnology,haslowdarkcurrentandrelativelysmallcapacitance(typically,about30nAand25pF,respectively,atroomtemperatureandagainof200).Theset-upusedforthemeasurementsisschematicallydepictedinFig.1.TheAPDconnectedtoalow-noisechargesensitivepreamplifier(CrematCR-101D)wasmountedinsideametalliccageandplacedintoaliquidnitrogencryostat.Lightpulsesof0.5μsindurationwereproducedbyagreenLEDdrivenbyapulser.TheLEDwasmountedinablackboxoutsidethecryostatandopticallyconnectedtotheAPDthroughanopticfiber.TheintensityoflightpulsesgeneratedbytheLEDwasmonitoredbyaPMT(HamamatsuR1668).TheoutputsignaloftheAPDwasamplifiedbyachargesensitivepreamplifierfollowedbyaspectroscopyamplifier(Canberra2021).Semigaussianshapingwithshapingtimeof1μswasused.TheamplifiedandshapedsignalsfromboththeAPDandthePMTweredigitizedbyapeakADC(LeCroy2259B)andstoredinaPC-basedmultichannelanalyzer.ThePCalsocontrolledtheLEDandcalibrationpulsersand,synchronously,providedagatefortheADC.Thelinearity,amplification,pedestal(offset)andtheelectronicnoiseofthewholespectrometricchannelweremeasuredusingcalibrationpulsesfromahighprecisionpulserfedtotheinputofthepreamplifierthroughacapacitanceof2.2pF.Thecalibrationwasverifiedbyobservingthepulseheightspectrumduetothedirectconversionof60keVγ-rays,froman242Amsource,inthephotodiodeatunitarygain.ForprecisemeasurementoftheexcessnoisefactoritisveryimportanttomaintainthestabilityoftheAPDgainduringtheacquisition.Thisrequiresstablebiasvoltageand4temperatureasthegainstronglydependsonbothofthem,especiallyathighgainvalues.Inourmeasurements,thestabilityofthebiasvoltagewasbetterthan0.1V.ToreducetheripplesfromtheHVsupply(CAENN471),anRC-filterwithatimeconstantof1swasused.Thevariationsofthetemperatureweremeasuredwithaprecisionof0.02°Cusingaplatinumthermoresistorfixeddirectlytothephotodiode.Duringtheacquisitionofasingledatapoint,thetemperaturevariationdidnotexceed0.2°C.3.ExperimentalMethodsandResultsA.MeasurementofGainThegainofthephotodiodeasfunctionofthebiasvoltagewasmeasuredoperatingtheLEDinpulsemode.Foreachvalueofthebiasvoltage,thedistributionoftheamplitudeoftheAPDchargesignalswasrecorded.Themeanvalueofthedistributionwasnormalizedtothemeanamplitudeofthesignalsmeasuredatlowvoltage(Vbias=400V)wherethegainisunitary[7].SomeresultsareplottedinFig.2.B.MeasurementofExcessNoiseFactorInanavalanchephotodiode,therandomcharacterofthemultiplicationprocessoftheelectron-holepairsunderanelectricfieldintroducestheso-calledmultiplicationnoise,whichischaracterizedbytheexcessnoisefactor,F,definedas2222MmmmF==(1)wheremisthegainofanavalancheinthephotodiodeandmM