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Germanium detector studies in the framework of the GERDA experiment [Elektronische Ressource] / put forward by Dušan Budjáš

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DissertationsubmittedtotheCombinedFacultiesoftheNaturalSciencesandMathematicsoftheRupertoCarolaUniversityofHeidelberg,GermanyforthedegreeofDoctorofNaturalSciencesPutforwardbyDušanBudjášfromBratislavaOralexamination:6.5.20092 Germanium detector studies in the framework of the GERDA experiment Referees: Prof.Dr.WolfgangHampel Prof.Dr.WolfgangKrätschmer 34 Germanium-Detektor-Studien im Rahmen des GERDA-Experiments Das "GERmanium Detector Array" (GERDA) ist ein Experiment mit extrem niedrigemUntergrund,daszurzeitam"LaboratoriNazionalidelGranSasso"aufgebautwird.GERDAwird76nachdemneutrinolosenDoppelBetaZerfallvon Gesuchen,mitdemZieldieUntergrundeffekteumeinenFaktor100gegenüberdenVorgängerExperimentenzuunterdrücken.DiesesBestrebenmachtinnovativeDesignAnsätze,strengeAuswahlanMaterialienmitniedrigerEigenradioaktivitätundneueTechnikenzuraktivenUnterdrückungdesUntergrundesnotwendig.DaszentraleElement76von GERDA ist ein Array mit Geangereicherte GermaniumDetektoren für ionisierendeStrahlung.GermaniumDetektorensindauchdaszentraleThemadieserDissertation.

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Dissertation
submittedtothe
CombinedFacultiesoftheNaturalSciencesandMathematics
oftheRupertoCarolaUniversityofHeidelberg,Germany
forthedegreeof
DoctorofNaturalSciences









Putforwardby
DušanBudjáš
fromBratislava
Oralexamination:6.5.20092



Germanium detector studies
in the framework of the GERDA experiment

















Referees: Prof.Dr.WolfgangHampel
Prof.Dr.WolfgangKrätschmer
34 Germanium-Detektor-Studien im Rahmen des GERDA-Experiments
Das "GERmanium Detector Array" (GERDA) ist ein Experiment mit extrem niedrigem
Untergrund,daszurzeitam"LaboratoriNazionalidelGranSasso"aufgebautwird.GERDAwird
76nachdemneutrinolosenDoppelBetaZerfallvon Gesuchen,mitdemZieldieUntergrundeffekte
umeinenFaktor100gegenüberdenVorgängerExperimentenzuunterdrücken.DiesesBestreben
machtinnovativeDesignAnsätze,strengeAuswahlanMaterialienmitniedrigerEigenradioaktivität
undneueTechnikenzuraktivenUnterdrückungdesUntergrundesnotwendig.DaszentraleElement
76von GERDA ist ein Array mit Geangereicherte GermaniumDetektoren für ionisierende
Strahlung.GermaniumDetektorensindauchdaszentraleThemadieserDissertation.DerersteTeil
beschreibtdieImplementierung,dieTestsunddieOptimierungderMonteCarloSimulationenvon
GermaniumSpektrometern,diefürdieSelektionvonMaterialienmitniedrigerEigenradioaktivität
unabkömmlich sind. Die Simulationen sind wesentlich für die Auswertungen der Gamma
StrahlungsMessungen.DerzweiteTeilbeschäftigtsichmitderEntwicklungundderPrüfungeiner
Methode zur aktiven Unterdrückung des Untergrundes, die auf einer FormAnalyse des
GermaniumDetektorSignals besteht. Dies wurde zum ersten Mal für einen Detektor des BEGe
Typs, der eine kleine AusleseElektrode beinhaltet, verwirklicht. Als Ergebnis dieser Arbeit ist
BEGe nun als eine der beiden DetektorTechnologien in der Forschung und Entwicklung für die
zweitePhasedesGERDAExperimentsenthalten.EineUnterdrückungdesHauptuntergrundesfür
GERDA wird aufgezeigt, mit einer (0.93±0.08)prozentigen Überlebenswahrscheinlichkeit für
60 226 228 228CoEreignisse, (21±3)% für Ra und (40±2)% für Th. Die Akzeptanz von Th Doppel
EscapeEreignissen,dieanalogzuDoppelBetaZerfallsind,wurdeauf(89,2±0,9)%gehalten.
Germanium detector studies in the framework of the GERDA experiment
TheGERmaniumDetectorArray(GERDA)isanultralowbackgroundexperimentundercon
76
structionatLaboratoriNazionalidelGranSasso.GERDAwillsearchfor Geneutrinolessdouble
betadecaywithanaimfor100foldreductioninbackgroundcomparedtopredecessorexperiments.
Thisambitionnecessitatesinnovativedesignapproaches,strictselectionoflowradioactivitymate
rials,andnoveltechniquesforactivebackgroundsuppression.ThecorefeatureofGERDAisits
76
array of germanium detectors for ionizing radiation, which are enriched in Ge. Germanium
detectors are the central theme of this dissertation. The first part describes the implementation,
testing, and optimisation of Monte Carlo simulations of germanium spectrometers, intensively
involved in the selection of lowradioactivity materials. The simulationsare essential for evalua
tionsofthegammaraymeasurements.Thesecondpartconcernsthedevelopmentandvalidationof
an active background suppression technique based on germanium detector signal shape analysis.
ThiswasperformedforthefirsttimeusingaBEGetypedetector,whichfeaturesasmallreadout
electrode.Asaresultofthiswork,BEGeisnowoneofthetwodetectortechnologiesincludedin
researchanddevelopmentforthesecondphaseoftheGERDAexperiment.Asuppressionofmajor
GERDA backgrounds is demonstrated, with (0.93±0.08)% survival probability for events from
60 226 228 228
Co,(21±3)%for Ra,and(40±2)%for Th.Theacceptanceof Thdoubleescapeevents,
whichareanalogoustodoublebetadecay,waskeptat(89±1)%.
5
6 Contents
Introduction ....................................................................................................................................9
Motivation ........................................................................................................................................9
1. Physicsofneutrinos................................................................................................................10
1.1. NeutrinosandtheStandardModelofparticlephysics .................................................10
1.2. Openquestionsinthephysicsofneutrinos...................................................................11
1.3. Neutrinolessdoublebetadecay ....................................................................................12
2. Germaniumdetectors .............................................................................................................14
2.1. Fundamentalprinciplesofsemiconductordetectors.....................................................14
2.2. Radiationinteraction.....................................................................................................15
2.3. Signaldevelopmentinasemiconductordetector .........................................................17
2.4. HPGedetector...............................................................................................................18
2.5. Signalreadoutandresolution.......................................................................................19
3. TheGERDAexperiment ........................................................................................................20
3.1. DatatakingphasesofGERDA .....................................................................................22
3.2. LowbackgroundmaterialsselectionandHPGespectrometry.....................................23
3.3. Backgroundsuppressionusingpulseshapeanalysis....................................................24
I Germanium spectrometry: Monte Carlo simulations for low-level
measurement evaluation ......................................................................................................27
4. LowlevelgammarayspectrometryatMPIK........................................................................27
5. Evaluationofmaterialscreeningmeasurements ....................................................................27
6. MonteCarlocode ...................................................................................................................29
7. MonteCarlosimulationsofmaterialscreeningdetectors......................................................30
8. MonteCarlosimulationvalidationwithmeasurements.........................................................33
8.1. EnvironmentalRadioactivityComparisonExercise2005............................................33
8.1.1.ComparisonofGERDAlaboratories ..................................................................33
8.1.2.Results.................................................................................................................34
8.1.3.Additionalinvestigation......................................................................................35
8.1.4.Discussion...........................................................................................................36
8.2. EnvironmentalRadioactivityProficiencyTestExercise2007 .....................................36
8.3. AssessmentofBrunoandDariosoftwaremodels ........................................................38
9. OptimisationofthesoftwaremodelofCorradospectrometer ...............................................41
9.1. Experimentalmeasurements .........................................................................................42
9.2. Deadlayerthicknessdetermination ..............................................................................42
9.3. Detectoractivevolumedetermination ..........................................................................43
9.4. Resultsanddiscussion ..................................................................................................45
10. ApplicationoftheMonteCarloefficiencydeterminationtomaterialscreening
measurementsevaluation ...............................................................................................................45
7II Germanium detector signal analysis: BEGe detector study................................... 49
11. Detectorsetup ........................................................................................................................ 50
11.1.DetectorreadoutandDAQsystem.............................................................................. 51
11.2.Detectorcharacterisationandtesting............................................................................ 54
11.2.1.DAQsystemstability ....................................................................................... 54
11.2.2.Chargecollectionstudy .................................................................................... 56
12. Pulseshapeanalysis............................................................................................................... 60
13. Experimentalpulseshapediscrimination .............................................................................. 64
13.1.Pulseshapediscriminationmethod .............................................................................. 65
13.2.Cutfunctionuncertaintiesandfluctuation ................................................................... 68
13.3.Validationmeasurements ............................................................................................. 71
13.3.1.CoincidencesetupforsingleComptonscatteringmeasurements .................... 72
13.3.2.Coincidencemeasurementsevaluation............................................................. 77
13.3.3.Collimatedbeammeasurements....................................................................... 80
13.3.4.Validationmeasurementsdiscussion................................................................ 82
13.4.Resultsanddiscussion.................................................................................................. 83
14. FeasibilityofBEGedetectorsfordoublebetadecayexperiments ........................................ 87
Appendix........................................................................................................................................ 89
Bibliography ................................................................................................................................. 91
Acknowledgements..................................................................................................................... 97

8 Introduction
Theaimofthisthesiswastotakepartinthe GERDAexperiment[1]withtheuseofandby
improvementofgermaniumspectrometry.Atthebeginningoftheintroductorypart,abriefover
viewofneutrinophysicsanditshistorywillbegiven,concludingwiththesummaryofexperimen
talsearchforsomeofitsopenquestions.BeforedescribingthedetailsoftheGERDAexperiment,
basicsofgermaniumspectrometrywillberecapitulated.Afterwards,theconnectionsofindividual
parts of this work to various aspects of GERDA will be explained. The experimental work is
dividedintotwomainparts,eachrelatedtooneaspectofgermaniumdetectortechnology.
Motivation
Theexperimentalworkpresentedinthisthesiscontributestothedevelopmentoftechnologyfor,
and the implementationof GERDA, an experiment in fundamental neutrino physics. The rapidly
evolvingfieldofneutrinophysicsisgainingrelevanceinawiderangeofsubjects,fromelementary
particlephysics,whereitoriginated,allthewaytopossibleapplicationsinastronomyandgeology.
Neutrinosrepresenttheedgeofourcurrentknowledgeofparticlephysics.Thetheoreticalmodel
describingneutrinooscillationsisthefirstconfirmedtheoryofphysicsbeyondtheStandardModel
whichhasreignedparticlephysicsfordecades.Thisnewphysicsisneededtosolvequestionsof
cosmology and astrophysics, such as the matterantimatter asymmetry, the dark matter, and dark
energy,whichStandardModelcannotanswer.
Inastrophysics,neutrinoshavealreadyplayedamajorrolebyverifyingourunderstandingofthe
processestakingplaceinsidetheSun.OtherpowerfulsourcesofneutrinosinourGalaxyaresuper
novae,releasingapproximately99%oftheirenergyinarapidburstofneutrinos.Detectingthem
canimproveourunderstandingofsupernovae,andevenprovideanadvancewarningforfollowup
astronomicalobservations.Inaddition,neutrinosthemselvescanbeusedasanobservationaltool,
providinganewwindowtotheUniverse.Theyaretheonlyknownparticleswhicharenotsignifi
cantlyattenuatedbytheirtravelthroughtheinterstellarmedium,andsocanrevealsourceshidden
forotherobservationaltechniques.ProjectslikeIceCube[2]andKM3NeT[3]arethefirststepsin
theemergingfieldofneutrinoastronomy.
Onourhomeplanet,neutrinophysicsisbecoming equallyimportant.Detecting geoneutrinos
(antineutrinosfromthedecaysofuranium,thoriumandpotassiuminrocks)canexplorethetotal
contentofradioactiveisotopesintheEarth'sinterior.Thisinformationwillbeanexperimentalvali
dationofthegeologicalmodeloftheplanet,withconsequencesontheunderstandingofplanetfor
mationandevolution.Firstresults,althoughonlyrough,werealreadyobtainedbytheKamLAND
experiment[4].Extendedfuturepossibilitiesincludeapplicationsoutsideofscience,e.g.,neutrino
detectors for remote monitoring of nuclear reactors, and even employing the matter effects on
directedneutrinobeamstosearchformineraldepositsinEarth'scrust[5].
Experiments such as GERDA lead to enabling and expanding the wide spectrum of neutrino
physicsapplications,bystudyingthefundamentalpropertiesofneutrinos.Theunderlyingtechnol
9ogyofGERDAisthegermaniumspectrometry,asubjectthatisalreadywellestablishedinabroad
range of applications. Germanium detectors are used, e.g., in nuclear industry, environmental
monitoring,andinfundamentalphysics.TheresearchanddevelopmentinvolvedinGERDAhelp
alsotopushthelimitsofthistechnology.Evenafterseveraldecadesoftheirwidespreademploy
ment,newsignificantimprovements(suchas, e.g.,segmentation,ordigitalpulseshapeanalysis)
arecontinuouslybeingappliedtogermaniumdetectors.
1. Physics of neutrinos
Neutrinoswerepostulatedatthe endof1930by Paulias asolutiontotheproblemof energy
conservationintheβdecay.Energiesofelectronsemittedinβdecayhaveacontinuousspectrum,
whichisinapparentcontradictionwiththelawofenergyconservation,unlessanother,unseen,par
ticlecarriesawaythelostenergy.Fermidevelopedthetheoryfurtherinthe1930's.Hisdescription
ofthemassless,chargeless,weaklyinteractingleptonwassuccessfulinexplainingtheβdecayand
othernaturalphenomena.However,theexperimentalconfirmationoftheexistenceofneutrinoswas
elusiveuntil1956,whenCowanandReinesobservedreactorantineutrinosintheSavannahRiver
experiment[6].
1.1. Neutrinos and the Standard Model of particle physics
TheStandardModel(SM)ofparticlephysicsdescribesneutrinosasmasslessleptons,comingin
threeflavours:e,,andτ,associatedwiththeirchargedleptons:electron,muon,andtau.Neutrinos
interactonlyviatheweakforce(WorZbosonexchange)andgravity.Neutrinosalwayshaveleft
handedhelicityandantineutrinosrighthandedhelicity.AccordingtotheSM,theleptonnumberis
conserved for each flavour family (i.e. the number of leptons of a particular flavour is the same
beforeandafteranyinteraction).
Indications that the SM description of neutrinos might be incomplete first appeared in 1968,
whentheHomestakeexperiment[7]discoveredadeficitinthemeasuredsolarneutrinofluxcom
paredtopredictionsfromtheastrophysicalmodeloftheSun.This"solarneutrinoproblem"[8]was
laterreinforcedbyseveralexperiments,includingKamiokande[9]andGallex/GNO[10,11].Later,
atmospheric neutrino experiments, detecting neutrinos from interactions of cosmic rays with the
Earth'satmosphere,observedsimilaranomalies(reportedbyIMB[12],Soudan2[13]andKamio
kande[14]collaborations).
TheneutrinooscillationhypothesiswasfirstsuggestedbyPontecorvoanddevelopedbyhimin
1960'sintoitsmodernform[15].Strongevidenceforneutrinooscillationsasasolutiontothesolar
neutrino problem was presented by the Sudbury Neutrino Observatory [16], which was able to
measurethesolarfluxofallneutrinoflavoursviaelasticscatteringandneutralcurrentinteraction,
while simultaneously monitoring the electrononly flavour flux via chargecurrent reaction. The
SuperKamiokandeexperimentprovidedfurtherevidencefortheatmosphericneutrinooscillations
[17],laterconfirmedbyacceleratorexperiments(K2K[18]andMINOS[19]).Finally,in2004the
reactor neutrino oscillation parameters measured by the KamLAND experiment, confirmed that
neutrinooscillationswithlargemixinganglearethesolutiontothesolarneutrinoproblem[20].
10