the future of cdms - kicp workshops€¦ · geodm cdms ii 3” x 1cm ~ 0.25 kg/det 16 detectors = 4...
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TheFutureofCDMS
ScottFallows
UniversityofMinnesota
GLCWXChicago,14June2010
CDMSOverview(I)
• CDMS=CryogenicDarkMatterSearch
• HalfamileundergroundattheSoudanUndergroundLabinnorthernMinnesota
• AttemptsdirectdetectionofWIMPsthroughelasticscatteringonnucleiinGecrystals
• Detectorscontinuouslycooledindilutionrefrigeratortomaintainoperationaltemperatureforsuperconductingphononsensors
CDMSOverview(II)
• SolidGe“ZIP”detectorsmeasureionizationandphononsignalsfromparticleinteractions
• Basicstrategy:1. Discriminate
electronrecoils(γ,βbackground)fromnuclearrecoils(WIMPsignal,n0background)
2. Removeneutronbackground
DetectorBasics• Ionization:
– Electron‐holepairsfrominteractionseparatedbyappliedEfield,andcollectedatopposingelectrodes
• Phonons:– collectedbyAl,causing
diffusionintosuperconductingtungstenTES,drivingitnormal
– ChangeinRTESamplifiedbySQUIDsandreadout
Al
quasiparticletrapAlCollector
WTransition‐EdgeSensor
Ge
quasiparticlediffusion
phonons
T(mK)Tc~80mK
R TES(Ω
)
4
3
2
1
~10mK
IonizationYield
• Ratioofionizationtophononenergyisgreaterforelectronrecoilsthanfornuclearrecoils
• Allowsustorejectmoste‐,γbackgroundsto1:104levelforCDMS‐IIZIPs
SurfaceEvents• Incompletechargecollectionforeventsonouter
10‐μm“deadlayer”
• Suppressedionizationyield
• Phononpulsesshowdifferenttimingproperties
Surfaceevents
Surface‐EventRejectionCalibrationData
Keep
Reject
Phononpulse‐shapecontainsinformationoninteractiondepth
Rejectionpowerof1:350forsurfaceeventsonCDMS‐IIZIPs
(M.Fritts)
8
n0Background:ShieldingandTagging
Scintillatorvetocage:muon‐taggingefficiency>99.9%
Polyethyleneshieldsagainstexternalradiogenicneutrons
Cosmogenicbackgroundestimatedtobe0.04eventsforlastrun;similarforradiogenic
PlasticScintillator
Polyethylene(40cm,10cm)
Lead,AncientLead(23cm,4cm)
CopperCans(afewcm)
713mrock2090mwe
Rockreducesmuonfluxby5x104
≈1µ/minuteincidentonscintillatorcage
SoudanUnderground
Lab
ScalingUp
• Ge‐basedtechnologyisoneofseveralcandidatesforton‐scaledarkmattersearches– competingwithliquidXe,liquidAr,andothers
• Increaseexposurewhilelimitingbackground– increaseddetectorsize,number
– improveddetectordesign,analysistechniques– improvedhandling:lowercontamination(fewerβ)
– deepersitetolimitneutronbackground(SNOLAB)
mZIP• Relativelyminordesignchanges
• 1”thick,2.5xsuppressionofsurfaceevents
• “Stadium”phononsensordesign:
– Coversmoresurfacearea
– Improvesphononcollection
• “Mercedes”‐likephononsensorlayout
– Betterphononsignalattheouteredge
– Breaksdegeneraciesinpositionreconstruction
– Improvesphonontiminginformation
mZIPatSoudan
• SuperTower1installedatSoudan– Five1”thickmZIPs+twoendcapvetodetectors
– Tookdatafall/winter2009‐10– Dataanalysisongoing
iZIP• Majordesignchange
• Still3”x1”(650gGe)
• Phononsensors:onbothsidesforfull3Dreconstruction
• Chargeandphononchannelsinterleaved
iZIPChargeCollection
• Chargelinesalternatingat±2V/0Vcauseasymmetrice‐/h+collectionforsurfaceevents
• Chargecollectionissymmetricforbulkevents
• Vastlyreducesdominantbackground
e‐
h+
e‐h+h+ +g+g
e‐h+h+h+
e‐e‐e‐
ChargeAsymmetry
• Witha109Cdsourceontheelectron(+2V)side,holecollectionisreducedforsurfaceevents–theygotothee‐sideground
• 90%efficientq‐symmetrycutrejectssurfaceeventsto<1:1000
• Bonus:10xsmallerleakageoflowyieldsurfaceeventsintoNRbandevenbeforethiscut(strongertransverseEfields)
~100Hz109Cd
Holes
Electrons
Electrons
Holes
ReconstructioninPhonons
• Twoadditionalparameterstocombinewithq‐symm.1. Phononenergypartition(shown,center)
2. Timingdifferencebetweenrisingedgeofphononpulsesonupper/lowerdetectorfaces
• iZIPER/NRdiscriminationpowersatisfiesrequirementsofton‐scaleexperiment
θzpθzq
=
Phononenergy:sourceside
Phon
onene
rgy:otherside
θzp
θ zq
+
FromCDMSIItoSuperCDMStoGEODM
CDMSII3”x1cm~0.25kg/det16detectors=4kg~2yrsoperation(lastanalysis)
SuperCDMS3”x1”~0.64kg/detSoudan SNOLAB25detectors=15kg 150detectors=100kg2yrs~10,000kg‐d 3yrs~100,000kg‐d
SuperCDMSSNOLABandGe‐ObservatoryforDarkMatter(GEODM) 6”x2”~5.1kg/detSNOLAB DUSEL20detectors=100kg 300detectors=1.5ton3yrs~100,000kg‐d 4yrs~1.5Mkg‐d
(V.Mandic)
Summary
• NewiZIPdetectorsoffervastlyimprovedperformance– Chargesymmetry
– ReducedlowyieldsurfaceeventsinNRband– Phononenergypartition– Improvedphonontimingdiscrimination
• CombinedwithreducedneutronbackgroundatSNOLAB/DUSEL,Gedetectorsshouldbecompetitiveinton‐scaleWIMPsearch
Backup:HowDoWeGetThere?
• Currentsurface‐eventrejectionefficiency:3×10‐3.• SuperCDMS‐15kg(Soudan):
– x2(cuts)x2.5(1”thick)x1.6(lowerbkgd)⇒10x.• SuperCDMS‐100kg(SNOLAB):
– x2(lowercontamination)x2(electrodes)x2(phonontiming)x2(phononcollection)⇒150x.
• GEODM1.5ton(Homestake‐7400):– x2(2”thick)x2(lowercontamination)x3(detectorimprovements)⇒2000x.
• BulkEMbackground(gammas):similarfactorstoabove.• Lowercontamination(radonmitigation,materialscreening).• Neutronbackground:
– Deepersites(SNOLAB,Homestake‐7400).– Internalshielding(insidecryostat).– Externalmoderator(waterorpoly).
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