NeutronBackgroundsinDarkMa3erExperiments

AndreasBiekertPhysics290E

October26,2016

Background:h3ps://upload.wikimedia.org/wikipedia

•  Electronrecoil(ER)eventshavemuchlargercountratethannuclearrecoil(NR)events.

•  PossibletodiscriminatebetweenERandNR.

•  NeutronNReventscanmimicWIMPsignal.

BackgroundsandSignalDiscriminaPon

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BlueeventsfromERcalibraPon(133Ba)andGreeneventsfromNRcalibraPon(252Cf)inCDMSZIPdetector.From[1]

UnderstandingNeutronBackgroundsinExperiments

•  Neutronbackgroundscanbeverysite,detector,andenvironmentspecific.

•  Canbedifficulttomeasureaccurately.•  UnderstandingcomesfromcarefulconsideraPonof:– Theory(relevantsources,distribuPons,etc.)– SimulaPon(commonlywithGEANTand/orFLUKA)– Measurements(ratecounPng)

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MainSourcesofNeutrons

•  Lowenergyneutronsfrom(α,n)reacPonsandspontaneousfissioninsurroundingrockanddetectorcomponents

•  HighenergyneutronsfrommuoninteracPonsinsurroundingrockanddetectorcomponents

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(α,n)reacPons

•  238U,235U,and232Thdecaychainsproduceα’s.

•  Differentisotopeshavedifferentenergycutoffsfor(α,n)reacPons

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238U:From:h3ps://upload.wikimedia.org/wikipedia/

(α,n)reacPons

•  238U,235U,and232Thdecaychainsproduceα’s.

•  Differentisotopeshavedifferentenergycutoffsfor(α,n)reacPons

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235U:From:h3ps://upload.wikimedia.org/wikipedia/

(α,n)reacPons

•  238U,235U,and232Thdecaychainsproduceα’s.

•  Differentisotopeshavedifferentenergycutoffsfor(α,n)reacPons

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232Th:From:h3ps://upload.wikimedia.org/wikipedia/

DecayChainRelevancetoNeutronBackgrounds

From[2]

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CRESSTGranSassoNeutronBackgroundAnalysis(From[2])

•  ExperimentmovedfromHallAtoHallCbetweengeneraPons.

•  Prompteddetailedneutronbackgroundstudy.

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Right:h3p://www.nature.com/polopoly_fs/7.4565.1337781034!/image/gran-sasso-graphic-on-page.jpg_gen/derivaPves/landscape_630/gran-sasso-graphic-on-page.jpgLek:h3ps://www.mppmu.mpg.de/english/cresst_image4.gif

CRESSTGranSasso(α,n)SimulaPons

•  BackgroundratesspecifictorockfoundaroundHallAatGranSasso•  ContribuPonlevelsoffaker~1mofrock/concrete

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SpontaneousFission

•  Nucleusdecaysintotwodaughternucleiand2-3neutrons.

•  Possibleinprincipleforourfriends238U,235U,and232Th.

•  InpracPce,onlyappreciablecontribuPonisfrom238U.

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CRESSTGranSassoTotalNeutronFluxComparison

•  CarefulsimulaPonscanreproduceexperimentaldata(measurementperformedinhallA)

•  SignificanceofwatercontenttoneutronfluxraisesquesPonsaboutannualmodulaPonofsignalseenbyDAMA–  DAMA’ssignalpersistsandnotconclusivelyexplained

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PassiveShielding

•  Usuallymadeof:– Wax– Hydrocarbon(Polyethylene,scinPllator,etc.)

– Water•  A3enuatesneutronfluxfromexternalsources.

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EffectofwaxshieldingaroundearlyCDMSexperimentatStanfordUndergroundFacility.Leadshielddesignedtoreduceϒfluxindetector.From[3]

CDMSStanfordUndergroundFacilityNeutronBackgroundAnalysis

•  Imagesfrom[4]•  Experiment(CDMSI)conductedatmuchshallowerdepth.•  BackgroundanalysisonneutronsoriginaPngfrommuon

interacPons.

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NeutronproducPonprocessesbymuons

•  Muoncapturefollowedbyneutronemission.Fallsoffaker300meterwaterequivalent

•  DirectmuonspallaPonejectsneutron

•  NeutronsproducPonbyhadronsfrommuon-generatednuclearshowers

•  Neutronsproducedbygammasinmuon-generatedelectromagnePcshowers

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MuonInteracPonsinDetectorComponents

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•  Passiveshieldingworksonneutronsgeneratedbymuonsexternaltothedetector(forthemostpart).

•  MuonscangenerateneutronsinsidedetectorbyinteracPngwithdetectorcomponents.

Leadshielding: CopperCrysotat:Plotsfrom[3]

MuonInteracPonsinDetectorComponents

•  PlotshowsneutronfluxresulPngfromdifferentmuoninteracPonprocesses(280GeVmuons).

•  DifferentdetectorcomponentshavedifferentsensiPviPes.

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From[5]

AcPveVetoLayer•  Modernexperimentshave

vetolayersaroundexperiments.–  LUXusedwaterwithPMTsthatmonitorforCherenkovradiaPonfrommuons.

–  CDMSIIusedplasPcscinPllatorforsimilarpurpose

•  Vetolayerscanalsodetectsomehighenergy(“punch-through”)neutronsfromexternalmuoninteracPons

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Lek:LUXwatershield.From[6]

Right:CDMSIIplasPcscinPllatorveto.From[1]

AddiPonalStrategies

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•  VetoingNReventswithmulPplesca3eringsinthedetector– WIMPsnotlikelytomulPplesca3erinthedetector

•  VetoingNReventswithaccompanyingERevents–  ThesearetheresultofneutrondeepinelasPcsca3ering

•  Cleanerandcleanerdetectormaterials

SimulaPonofneutronsca3eringmulPplicityinagenericliquidxenondetectordeepunderground.From[7]

LUXNeutronBackgrounds[8]•  Neutronsourcesarepreviouslydiscussed(α,n)andspontaneousfissionprocessesoccurringin:–  PMTs– AccumulaPonof210Pbdaughtersondetectorsurfaces(negligible)

–  Cryostats(30%ofbaselinePMTesPmate)•  Punch-throughneutronsfrommuoninteracPonsinrock(30%ofbaselinePMTesPmate)

•  FinalesPmatetranslatesto3x10-4NReventsperdaypassingallcuts

•  NReventssubdominantbackground

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CDMSIINeutronBackground[1]

•  Unvetoedmuon-inducedNReventsforrun– 0.214±0.0081(stat.)±0.0086(syst.)forGedetectors– 0.129±0.0058(stat.)±0.0033(syst.)forSidetectors

•  Detectorcomponent-generatedneutroneventsforrunalsomuchlessthan1.

•  NRbackgroundenPrelysubdominant.

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NewExperiment–NuclearEmulsion(From[9])

•  Authorsassumepassivescreeningtechniquesforexternallygeneratedneutrons.

•  DetailedcalculaPons,simulaPons,andacPvitymeasurementstoesPmateintrinsicneutronbackground

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References[1]S.M.Fallows.MeasurementofNuclearRecoilsintheCDMSIIDarkMa8erSearch.PhDthesis,UniversityofMinnesota,December2014.[2]H.R.T.Wulandari.StudyOnNeutron-InducedBackgroundintheDarkMa8erExperimentCRESST.PhDthesis,InsPtutfürAstro-TeilchenphysikderTechnischenUniversitätMünchen,July2003.[3]A.D.Silva,etal.Neutronbackgroundforadarkma3erexperimentatashallowdepthsite.NuclearInstrumentsandMethodsinPhysicsResearchA,354:553–559,August1995.[4]T.Perera.TheLimiDngBackgroundinaDarkMa8erSearchatShallowDepth.PhDthesis,CaseWesternReserveUniversity,January2002.[5]H.M.Araùjo,etal.Muon-inducedneutronproducPonanddetecPonwithGEANT4andFLUKA.NuclearInstrumentsandMethodsinPhysicsResearchSecPonA:Accelerators,Spectrometers,DetectorsandAssociatedEquipment,545(1-2):398–411,June2005.[6]S.Fiorucci.TheLUXdarkma3ersearch.DarknessVisible:DarkMa3erinAstrophysicsandParPclePhysics,August2010.[7]M.J.Carson,etal.Neutronbackgroundinlarge-scalexenondetectorsfordarkma3ersearches.AstroparPclePhysics,21(6):667–687,Sep2004.[8]D.C.Malling.MeasurementandAnalysisofWIMPDetecDonBackgrounds,andCharacterizaDonandPerformanceoftheLargeUndergroundXenonDarkMa8erSearchExperiment.PhDthesis,BrownUniversity,May2014.[9]A.Alexandrov,etal.IntrinsicneutronbackgroundofnuclearemulsionsfordirecPonaldarkma3ersearches.arXiv,June2016.

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OtherSourcesP.Sorensen.SensiPvityandbackgroundsoftheluxdarkma3ersearch.APSAprilMeePng,2008.T.Classen.TheLUXWIMPdetector.24thPacificCoastGravityMeePng,March2008.K.Ni.DevelopmentofaLiquidXenonTimeProjecDonChamberfortheXENONDarkMa8erSearch.PhDthesis,ColumbiaUniversity,2006.M.J.Carson,etal.Vetoperformanceforlarge-scalexenondarkma3erdetectors.NuclearInstrumentsandMeth-odsinPhysicsResearchSecPonA:Accelerators,Spectrometers,DetectorsandAssociatedEquipment,548(3):418–426,August2005.

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