低能量阈高纯锗探测器用于暗物质探测实验研究

󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁

HIGH

ENERGY

PHYSICS

AND

NUCLEAR

PHYSICS

Vol.31,No.6

Jun.,2007

StatusofULE-HPGeDetectorExperimentfor

DarkMatterSearch*

LIXin1

YUEQian1;1)KIMS.K.2

LIYuan-Jing1

LIJin1

HEDao1

KWAKJ.W.2

WANGH.T.3

1(DepartmentofEngineeringPhysics,TsinghuaUniversity,Beijing100084,China)

2(SchoolofPhysics,SeoulNationalUniversity,Seoul151-742,Korea)

3(InstituteofPhysics,AS,Taipei11529,China)

AbstractAnUltraLowEnergyHPGe(ULE-HPGe)detector,withCsI(Tl)activeshielding,isappliedtothedirectdetectionexperimentforweaklyinteractingmassiveparticles(WIMP),andlocatedinKorea.Thesetupsforthewholesystemandthecalibrationhavebeencompletedandmorethanhalfayear’sbackgrounddatahavebeenaccumulated.Someexternalneutronandgammasourceexperimentswerecarriedouttostudytheoriginofthebackground.Theanalysisandpreliminaryresultsareshown,andanattractivefutureisalsoprovided.

Keywordsdarkmatter,WIMP,ULE-HPGedetector,calibration,vetoefficiency

1Introduction

Thedetectionofdarkmatterwhichistobe

detectors,thelowmassWIMPspacehasnotbeenscannedbytheseexperiments.Onenewexperimenthasbeenestablishedfornon-baryonicdarkmattersearchwithULE-HPGedetector,whichmainlyfo-cusesonthelowmassregionofdarkmattermassspectrum.Inthispaper,somedetailsofourlowmassdarkmatterexperimentwillbedescribedandapre-liminaryresultgiven.ThetargetforthisexperimentwilldetectdirectlytheWIMPwith1kgmulti-channelULE-HPGedetectorandasthefirststep,one5gpro-totypeULE-HPGedetectorhasbeenestablishedtostudythisfeasibility.

thoughtasthemajormasspartofourmatterworldhasbeenoneofthehighlighttopicsforparticlephysics,astronomyphysicsandcosmology.Accord-ingtosometheoreticalandexperimentalresults,themajorpartofdarkmatteriscomposedofnon-baryonicparticles.WIMPisoneofthemostattrac-tivecandidatesofnon-baryonicparticles.Directde-tectionofWIMPcangiveustheclearerimageabouttheinteractionbetweendarkmatterparticlesandnu-clei,whichwillbehelpfulforourunderstandingoftheessenceofdarkmatter.

ThetypicalWIMPsearchexperimentssuchasDAMA,CDMSandUKDMChavegiventheirexper-imentalresults,whichturntobethestringentexclu-sivecurvesintwodimensionsoftheWIMP-nucleuselasticscatteringcross-sectionvs.

WIMPmass.

[2]

[1]

2Experimentalsetup

Theultra-low-energyprototypeofa5gHPGe

detectorhasbeeninstalledat700m-deepY2L(YangyangUndergroundLaboratory)inKorea.Fig.1(a)showstheHPGedetector,withbothpas-siveandactiveshielding.Theshieldingmaterials

Duetotherelativelyhigherenergythresholdofthe

󰀃6󰀁󰀁󰀁󰀂󰀁󰀁󰀁󰀂󰀁󰀁󰀁󰀁󰀁󰀁󰀂󰀂󰀃󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀂565

566󰀁󰀁󰀁󰀃󰀁󰀁󰀁󰀃(HEP&NP)󰀃31󰀁

p(2)

1(ADCch/keV)

.

Fromfittingresults,thelinearityofbothhighgainandlowgainchannelsisacceptable,buttheoffsetofcalibrationequation,whichisthecorrespondingen-ergyvaluewhenthepulseheightisassumedatzero,isveryobvious.Inthehighgaincalibrationequation,thevalueofp0is−43.851andp1is387.617,whileinthelowgaincalibrationequationp0is2.18559andp1is31.4929.Forthelowgainchannel,−69.4eVcanbeadmittedwithalargeenergyregionabout100keV,butinthehighgainchannel,113.13eVhasbeencom-parabletoourexpectedthreshold,whichisobservedandmeasured,butnotcomprehended.

ThecalibrationforCsI(Tl)vetodetectorisalsocompleted,andfoursources,

109

Cd(AgX-ray),

241

Am,

238

U(decayedγ-rayfromthedaughteriso-tope234

Th)and57

Co,areusedtocalibrateit.Even

thoughCsI(Tl)crystaldetectorisonlyusedasac-tiveshielding,whichmeansthatitisenoughwhethertheCsI(Tl)channelgivesoutarealvetosignalcanbediscriminated,infact,theenergylinearityandoff-setaresatisfactoryforthisCsI(Tl)activeshieldingdetector.

4Backgrounddataanalysis

Sincethewholesystemsetupandcalibrationwere

completedinOctober2005,thebackgrounddatatak-inghasbeenrunformorethanhalfayear.Exceptthetimespentonotherresearchexperiments,about155days’datahavebeenaccumulated.ButtheHPGemassissosmall,5g,soitisonly7.75kg•day’sdatafromwhichthepreliminaryresultsareconcluded.Duringthedataacquisition,thereissomeelec-tronicnoise,withobviouslargeminusshootsinallchannels,whilethenormalpulseshouldbepositiveafterFADCdigitization.Sotheelectronicnoisecanberejectedeasily,evenwiththeonlineDAQprogram.Meanwhile,intheCsI(Tl)crystal,somesharpevent,withitspulseshapesimilartosinglephotonresponse,appearsfrequently.Thiskindofscintillationnoisecanberejectedwithpulseshapediscriminationmeth-ods.MeantimeisusedasthePSDparameterandIiisthecurrentamplitudecorrespondingtothetime

ti:

󰀁

Iiti

¯t

=i

󰀃6󰀁󰀁󰀁󰀂󰀁󰀁󰀁󰀂󰀁󰀁󰀁󰀁󰀁󰀁󰀂󰀂󰀃󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀂567

568󰀁󰀁󰀁󰀃󰀁󰀁󰀁󰀃(HEP&NP)󰀃31󰀁

󰀃6󰀁󰀁󰀁󰀂󰀁󰀁󰀁󰀂󰀁󰀁󰀁󰀁󰀁󰀁󰀂󰀂󰀃󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀂569

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(󰀁󰀁󰀁.󰀁󰀂󰀁󰀁󰀁󰀃󰀁󰀁,2004,28(8):877—880)4LIUYan,CHENCP,LIHBetal.NIM,2002,A482:125—143;YUEQianetal.HEP&NP,2002,26(7):728—734(inChinese)

(󰀁󰀁󰀁.󰀁󰀂󰀁󰀁󰀁󰀃󰀁󰀁,2002,26(7):728—734)

󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀂󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁󰀁*

󰀁󰀁

1

󰀁󰀁

1;1)

󰀁󰀂󰀁

1

󰀁󰀃

1

󰀁󰀁

1

󰀃󰀁󰀁

100084)151-742)11529)

2

󰀂󰀁󰀂

2

󰀁󰀁󰀂

3

1(󰀁󰀂󰀂󰀂󰀄󰀁󰀁󰀁󰀁2(󰀂󰀁󰀂󰀂󰀂󰀂󰀁󰀁󰀁3(󰀂󰀃󰀂󰀁󰀁󰀃󰀃󰀁

󰀁󰀁󰀂󰀁󰀁󰀁

󰀁󰀁󰀃󰀂󰀃󰀅󰀅󰀃󰀁󰀄󰀉󰀃󰀂󰀁󰀄󰀂󰀁󰀁󰀂󰀁󰀁󰀄󰀃WIMP󰀁󰀇󰀁󰀁󰀄,󰀆󰀂󰀁󰀂󰀁󰀃󰀁󰀁󰀁󰀃󰀁󰀇󰀃,󰀂󰀃󰀅󰀂󰀁󰀂󰀂󰀁󰀃󰀂󰀁󰀇󰀃󰀁󰀂󰀅󰀇󰀂󰀂󰀂.󰀁󰀂󰀁󰀃󰀅󰀂󰀁󰀁󰀃󰀃󰀆󰀄󰀃󰀁󰀂,󰀄󰀂󰀄󰀃󰀁󰀂󰀁󰀂155d󰀁󰀁󰀃󰀃󰀅.󰀃󰀁󰀂󰀆󰀂󰀁󰀃󰀁󰀁󰀄󰀂󰀁󰀁󰀂󰀁󰀁󰀁,󰀅󰀁󰀈󰀁󰀃󰀁󰀁󰀁.󰀇󰀄󰀁󰀃󰀅󰀁󰀂󰀅󰀁󰀃󰀅󰀆󰀄,󰀄󰀃󰀁󰀁󰀁󰀁󰀄󰀁󰀁󰀁󰀄.

󰀁󰀁󰀁󰀁󰀄󰀃WIMPs󰀂󰀁󰀃󰀁󰀁󰀁󰀃󰀁󰀇󰀃

󰀃󰀆󰀁󰀃󰀂󰀁󰀂