Table Of Content

CUORE: A CRYOGENIC UNDERGROUND OBSERVATORY FOR RARE EVENTS 7th February 2008 5 0 0 Dipartimentodi Fisicadell’Universitàdi Milano-Bicocca 2 eSezionedi Milanodell’INFN,MilanoI-20126,Italy n a Dipartimentodi IngegneriaStrutturaledelPolitecnicodi Milano, J MilanoI-20133,Italy 6 DepartmentofPhysicsandAstronomy,UniversityofSouthCarolina, 1 v Columbia,South Carolina29208USA 0 Laboratori Nazionalidel Gran Sasso, 1 0 I-67010,Assergi(L’Aquila),Italy 1 0 DipartimentodiFisicadell’UniversitàdiFirenze 5 eSezione diFirenze dell’INFN, FirenzeI-50125, Italy 0 / x LawrenceBerkeley NationalLaboratory, e Berkeley,California94720,USA - p e LawrenceLivermoreNationalLaboratory, h Livermore,California,94551,USA : v i LaboratoriodeFisicaNucleary AltasEnergias, X UniversidaddeZaragoza, 50009Zaragoza, Spain r a KamerlingOnnesLaboratory,Leiden University, 2300RAQ, Leiden,TheNetherlands DipartimentodiScienzeChimiche,FisicheeMatematichedell’Universitàdell’Insubria e SezionediMilanodell’INFN, ComoI-22100, Italy Dipartimentodi Fisicadell’Universitàdi Genova eSezione diGenovadell’INFN, GenovaI-16146,Italy UniversityofCalifornia, Berkeley,California94720USA LaboratoriNazionalidi Legnaro, 1 2 I-35020Legnaro (Padova),Italy Dipartimentodi Fisicadell’Universitàdi Roma eSezionedi Roma1dell’INFN, RomaI-16146,Italy (TheCUORE Collaboration) Professor EttoreFiorini,UniversitàdiMilanoBicocca, Spokesman. 3 SCIENTIFIC PERSONAL MEMBERSOFTHE CUORE COLLABORATION R.Ardito1,2,C.Arnaboldi1,D.R.Artusa3,F.T.AvignoneIII3,M.Balata4,I.Bandac3, M.Barucci5,J.W.Beeman6,F.Bellini14,C.Brofferio1,C.Bucci4,S.Capelli1,F.Capozzi1, L. Carbone1, S. Cebrian7, C. Cosmelli14, O. Cremonesi1, R. J. Creswick3, I. Dafinei14, A. de Waard8, M. Diemoz14, M. Dolinski6,11, H. A. Farach3, F. Ferroni14, E. Fiorini1, G. Frossati8, C. Gargiulo14, E. Guardincerri10, A. Giuliani9, P. Gorla7, T.D. Gutierrez6, E.E.Haller6,11,I.G.Irastorza7,E.Longo14,G.Maier2,R.Maruyama6,11,S.Morganti14, S. Nisi4, E. B. Norman13, A. Nucciotti1, E. Olivieri5, P. Ottonello10, M. Pallavicini10, V. Palmieri12, E. Pasca5, M. Pavan1, M. Pedretti9, G. Pessina1, S. Pirro1, E. Previtali1, B. Quiter6,11, L. Risegari5, C. Rosenfeld3, S. Sangiorgio9, M. Sisti1, A. R. Smith6, S. Toffanin12, L. Torres1, G. Ventura5,N. Xu6, and L.Zanotti1 1. Dipartimento di Fisica dell’Università di Milano-Bicocca e Sezione di Milano dell’INFN, MilanoI-20126,Italy 2. DipartimentodiIngegneriaStrutturaledelPolitecnicodiMilano,MilanoI-20133, Italy 3. Dept.of Physics and Astronomy, University of South Carolina, Columbia, South Carolina, USA 29208 4. LaboratoriNazionali delGran Sasso, I-67010,Assergi (L’Aquila),Italy 5. DipartimentodiFisicadell’UniversitàdiFirenzeeSezionediFirenzedell’INFN, Firenze I-50125,Italy 6. LawrenceBerkeley NationalLaboratory,Berkeley, California,94720,USA 7. Laboratorio de FisicaNuclear y Altas Energias, Universidàdde Zaragoza, 50009 Zaragoza, Spain 8. KamerlingOnnesLaboratory,LeidenUniversity,2300RAQ,Leiden,TheNether- lands 9. Dipartimento di Fisica e Matematica dell’Università dell’Insubria e Sezione di Milanodell’INFN, ComoI-22100,Italy 10. DipartimentodiFisicadell’UniversitàdiGenovaeSezionediGenovadell’INFN, GenovaI-16146,Italy 4 11. UniversityofCalifornia, Berkeley, California94720,USA 12. LaboratoriNazionali diLegnaro,I-35020 Legnaro(Padova), Italy 13. LawrenceLivermoreNationalLaboratory,Livermore,California, 94550,USA 14. Dipartimento di Fisica dell’Universitàdi Roma e Sezione di Roma 1 dell’INFN, RomaI-16146,Italy CONTENTS 1 Contents 1 Executivesummary 10 2 CUOREScience Motivation 12 2.1 TheoreticalMotivationofNeutrinolessDouble-BetaDecay Experiments 12 2.2 NeutrinoMixingMatrix . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3 NeutrinolessDouble-BetaDecay . . . . . . . . . . . . . . . . . . . . . 13 2.4 ExperimentalProspects . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3 Status ofNeutrinoless Double-Beta Decay Experiments 19 3.1 MainstrategiesforDoubleBetaDecay searches . . . . . . . . . . . . . 21 3.2 Overviewofmainexperimentsand synopsisofresults. . . . . . . . . . 24 3.3 Thermaldetectorapproach todoublebetadecay . . . . . . . . . . . . . 27 3.4 Nextgenerationexperiments . . . . . . . . . . . . . . . . . . . . . . . 28 4 CUOREexperimental prospects 31 4.1 Doublebetadecay prospects . . . . . . . . . . . . . . . . . . . . . . . 33 4.2 OtherphysicswithCUORE . . . . . . . . . . . . . . . . . . . . . . . . 35 4.2.1 WIMP detection . . . . . . . . . . . . . . . . . . . . . . . . . 35 4.2.2 Solar axiondetection . . . . . . . . . . . . . . . . . . . . . . . 40 4.2.3 Searches for otherrare processes . . . . . . . . . . . . . . . . . 43 5 Principles ofoperationofTeO bolometers 47 2 5.1 Basicprinciplesofthermalparticledetectors . . . . . . . . . . . . . . . 47 5.2 Semiconductorthermistors . . . . . . . . . . . . . . . . . . . . . . . . 49 5.3 Detectoroperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 5.4 Bolometerapplications . . . . . . . . . . . . . . . . . . . . . . . . . . 51 5.5 Detectormodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 6 Previousresults ofMiDBD and CUORICINO 59 6.1 LNGScryogenicsetups . . . . . . . . . . . . . . . . . . . . . . . . . . 59 6.2 MiDBDexperiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 6.3 CUORICINO experiment . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.3.1 CUORICINO backgroundmeasurements . . . . . . . . . . . . 72 6.3.2 CUORICINO backgroundanalysis. . . . . . . . . . . . . . . . 72 6.3.3 AnalysisoftheCUORICINO backgroundabove3 MeV . . . . 80 6.3.4 Preliminary measurementsoftheCoppersurface contaminations 83 6.3.5 CUORICINO vs. CUORE . . . . . . . . . . . . . . . . . . . . 84 CONTENTS 2 7 CUOREproject 85 7.1 SingleCUORE detector . . . . . . . . . . . . . . . . . . . . . . . . . . 86 7.2 TheNTDthermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 7.3 ModularstructureoftheCUORE detector . . . . . . . . . . . . . . . . 94 7.4 Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 8 Cryogenicsystem 99 8.1 DevelopmentofaPulse-Tube-assistedcryostat . . . . . . . . . . . . . 101 8.2 Shieldingrequirements . . . . . . . . . . . . . . . . . . . . . . . . . . 102 9 Required laboratory 104 9.1 Coolingwaterrequirements . . . . . . . . . . . . . . . . . . . . . . . . 106 9.2 Powerrequirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 9.3 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 9.4 Spacerequirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 10 Electronics 107 10.1 General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 10.2 Thermistorbiasingand detectorwiring . . . . . . . . . . . . . . . . . . 108 10.3 Preamplifierand Cold bufferstage . . . . . . . . . . . . . . . . . . . . 110 10.4 Programmablegainamplifier . . . . . . . . . . . . . . . . . . . . . . . 112 10.5 Antialiasingfilterand analogtrigger . . . . . . . . . . . . . . . . . . . 112 10.6 CalibratingPulseGeneratorSystem . . . . . . . . . . . . . . . . . . . 112 10.7 VoltageSupplysystem . . . . . . . . . . . . . . . . . . . . . . . . . . 113 10.8 Digitalcontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 10.9 EMIShielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 10.10Noiseconsiderations . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 11 Dataacquisitionsystem (DAQ) 116 11.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 11.2 General architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 11.3 Readout board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 11.4 ComputingsystemandDAQ layout . . . . . . . . . . . . . . . . . . . 121 11.5 Slowcontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 12 Constructionprocedures 123 12.1 MaterialSelection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 12.1.1 Materials forcrystalgrowth . . . . . . . . . . . . . . . . . . . 123 12.1.2 Materials forthedetectorstructure . . . . . . . . . . . . . . . . 124 12.1.3 Shieldingmaterials . . . . . . . . . . . . . . . . . . . . . . . . 126 12.1.4 Othercomponentsoftheexperimentalsetup . . . . . . . . . . . 126 CONTENTS 3 12.2 Constructionenvironment . . . . . . . . . . . . . . . . . . . . . . . . . 126 12.3 Lowbackgroundtechnologytransfertothecrystalgrowthcompany . . 128 12.4 CUORE assemblyprocedure . . . . . . . . . . . . . . . . . . . . . . . 129 12.5 Workbreakdown andconstructionschedulefortheCUORE detector . . 133 12.6 General timescheduleoftheCUORE project . . . . . . . . . . . . . . 133 13 Off-lineanalysis 136 13.1 First-levelanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 13.2 Second-levelanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 14 Simulationand predicted performances 140 14.1 Background simulations . . . . . . . . . . . . . . . . . . . . . . . . . 140 14.2 Bulkcontaminations . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 14.3 Surface contaminations . . . . . . . . . . . . . . . . . . . . . . . . . . 143 14.4 Cosmogeniccontribution . . . . . . . . . . . . . . . . . . . . . . . . . 147 14.5 Undergroundneutron,µ and γ interactions . . . . . . . . . . . . . . . . 151 14.6 Twoneutrinosdoublebetadecay background . . . . . . . . . . . . . . 154 15 Innovationsinthe CUOREstructure 155 15.1 Innovationin thesinglemodule . . . . . . . . . . . . . . . . . . . . . . 155 15.1.1 Optimizationofthethermistorsensitivity . . . . . . . . . . . . 155 15.1.2 Innovativethermalcouplingsin detectorconstruction . . . . . . 156 15.1.3 Modificationofthebasiccrystal size. . . . . . . . . . . . . . . 156 15.2 PossibleinsertioninCUOREofcrystalscontainingcompoundsofother nuclearcandidatesforββ(0ν) . . . . . . . . . . . . . . . . . . . . . . 157 15.3 Developmentofsurface sensitiveTeO elements . . . . . . . . . . . . . 159 2 15.4 DevelopmentofscintillatingTeO bolometers . . . . . . . . . . . . . . 163 2 16 CleaningofCUOREstructure: crystals,frames and ancillaries 165 16.1 Surface cleaningtechniques . . . . . . . . . . . . . . . . . . . . . . . 165 16.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 17 Enrichment option 182 18 Structure ofthe collaboration 183 19 Coststatement 185 20 Conclusions 188 LISTOFFIGURES 4 List of Figures 1 Normal and inverted mass hierarchies schemes. Quasi-degeneratehier- archycorresponds tothecasem >> δm T >> δm . . . . . . . . . . 14 1 A S 2 Exclusion projected for 1 year of CUORE assuming a threshold of 10 keV, a low energy resolutionof 1 keV, and low energy background lev- els of 0.05 and 0.01 c/keV/kg/day respectively. The dashed line corre- spondsto theMiDBD result. . . . . . . . . . . . . . . . . . . . . . . . 37 3 The solid lines represent the sensitivity plot in the (m,σ) plane for CUORE, assuming a threshold of 10 keV, two years of exposure (1500 kgyear)andflatbackgroundsof0.05and0.01c/keV/kg/day. Ithasbeen calculated for δ2 = 5.6 (see the text). The sensitivity curve has been h i also calculated for a possible threshold of 5 keV with a background of 0.01c/keV/kg/day(dashedline). . . . . . . . . . . . . . . . . . . . . . 39 4 Best bound attainable with CUORE (straight line labelled "CUORE") comparedwithothers limits. . . . . . . . . . . . . . . . . . . . . . . . 42 5 Simplified model of a thermal detector (a) and schematic read-out of a resistivethermometer(b) . . . . . . . . . . . . . . . . . . . . . . . . . 48 6 TypicalloadcurveforathermistoratT=8mK)(a)andresistance-power curvesforathermistorat differentbasetemperatures(b). . . . . . . . . 50 7 leftpanel: evaluationof theoptimumpoint ofa bolometer;rightpanel: evaluationofoperationpointwhich maximizesthesignal/noiseratio. . . 52 8 Networkrepresenting thedetectormodel. . . . . . . . . . . . . . . . . 54 9 DetectorMeritCurve: simulation(solidcurve)andexperimentalpoints forthe20 detectorarray. . . . . . . . . . . . . . . . . . . . . . . . . . 56 10 Real pulses(full line)and simulatedpulses(line+points). . . . . . . . . 56 11 SchemeoftheMiDBDdetectors. . . . . . . . . . . . . . . . . . . . . . 60 12 AnU+Th calibration,sumspectrumofthe20detectors . . . . . . . . . 61 13 DistributionofthesingleMiDBDdetectorenergyresolutions(FWHM) onthe208Tl 2615keV line. . . . . . . . . . . . . . . . . . . . . . . . . 61 14 Background spectrum: thealphaparticleregion . . . . . . . . . . . . . 62 15 Scatter plot of coincident events between all possible MiDBD detector pairs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 16 MiDBD-I (continuous line) and MiDBD-II (dashed line)background spectraafteranticoincidencecut. . . . . . . . . . . . . . . . . . . . . . 65 17 Schemeofasurface αdecay. . . . . . . . . . . . . . . . . . . . . . . . 67 18 Total spectrum (in anticoincidence) in the region of neutrinoless DBD obtained with the twenty crystal array. The solid curves represent the bestfit (lowestcurve)andthe68 %and 90 % C.L. excludedsignals. . . 69 LISTOFFIGURES 5 19 The CUORICINO detector: scheme of the tower and internal roman lead shields (left), the 13 planes tower (centre), the 4 crystal module (topright)and the9crystal module(bottomright). . . . . . . . . . . . 73 20 Summed calibration spectrum (232Th source just outside the crysotat) fromall theoperating 5 5 5 cm3 and 3 3 6 cm3 crystals. . . . . . . 74 × × × × 21 Distribution of the single CUORICINO detector energy responses nor- malizedto1 kg ofTeO . . . . . . . . . . . . . . . . . . . . . . . . . . 74 2 22 DistributionofthesingleCUORICINOdetectorenergyresolutions(FWHM) at the208Tl 2615keV line. . . . . . . . . . . . . . . . . . . . . . . . . 75 23 Summed background spectra from the operating 5 5 5 cm3 and (nat- × × ural abundance)3 3 6 cm3 crystals. . . . . . . . . . . . . . . . . . . 75 × × 24 Comparison between the background of the 5 5 5 cm3 crystals and × × thatofthenatural3 3 6 cm3 crystalsinthegammaregion. . . . . . . 76 × × 25 Comparison between the background of the 5 5 5 cm3 crystals and × × thatofthenatural3 3 6 cm3 crystalsinthealfaregion. . . . . . . . . 77 × × 26 Comparison between the background spectra of CUORICINO crystals andthebackgroundmeasured inMiDBD-IIexperiment. . . . . . . . . 77 27 Summedbackgroundspectrum(5.29kg y)fromalltheoperatingcrys- × talsintheregionofneutrinolessdoublebetadecayof130Te(Q-value=2528.8 keV). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 28 Comparisonbetweenspectraobtainedwithdifferentlinearizationmeth- ods. Theappearenceofclearalphastructureswiththepowerlawmetod isevident. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 29 Comparison between Monte Carlo and CUORICINO anticoincidence (top) and coincidence (bottom) spectra in the case of the TeO crystal 2 surfacecontaminations(λ 1 µm)specified in thefigure. . . . . . . . . 81 ∼ 30 Comparison between Monte Carlo and CUORICINO anticoincidence (top) and coincidence (bottom) spectra when a Th surface contamina- tionofthedetectorcopperholder(greenline)isadded(pinkline)tothe crystalconatminationsconsideredin fig. 29. . . . . . . . . . . . . . . . 82 31 TheCUORE detector(left), oneofthe19 towers(right). . . . . . . . . 85 32 Effects of the system thermal instabilities on the worsening of the de- tectorenergyresolution(above). Comparisonbetweencalibrationspec- tra before (black curve: ∆EFWHM = 30 keV) and after (red curve: 2615 ∆EFWHM =6 keV)thestabilizationprocedure. . . . . . . . . . . . . . 88 2615 33 Graph depicting the solution for thermal and epithermal flux for a set of three monitors, each with different ratios of thermal and epithermal cross-sections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 34 Resistivityofvariousbatches ofNTDGermaniumas afunctionof1/√T. 92 LISTOFFIGURES 6 35 Thermistorperformanceasafunctionofthecalculatednetacceptorcon- centration. The first six points (from the left) involved only one irradi- ation (and show a relatively large spread in deviations from the line) whilethelastthreepointsarefollowingthesecondirradiationandhave averysmallspread from theline. . . . . . . . . . . . . . . . . . . . . . 93 36 A fourdetectormodule. . . . . . . . . . . . . . . . . . . . . . . . . . . 95 37 Improvement in the Detector Merit Curve by increasing the thermistor size: 3 3 1 mm3 thermistors(stars)vs. 3 1.5 0.4 mm3 (diamonds). . 95 × × × × 38 Noise Power Spectra without damping suspension (1), with damping suspensiononly(2)and withdumpingsuspension+cold electronics(3). 96 39 Fifteen thermistorload curvesforthe20crystal array. . . . . . . . . . . 97 40 FinallocationofCUOREinhallAoftheundergroundLaboratoriNazion- alidel Gran Sasso. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 41 CUORE cryostat andshielding. . . . . . . . . . . . . . . . . . . . . . . 100 42 Temperatureinstabilityas observedina typicalMiDBDmeasurement. . 101 43 SchemeoftheCUORE setupinstallation. . . . . . . . . . . . . . . . . 105 44 Schematic diagram of the front-end electronic circuit operated at room temperature(WFE). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 45 Schematic diagram of the front-end having the first stage, the differen- tialbufferQ ,Q , atcold. . . . . . . . . . . . . . . . . . . . . . . . . . 110 1 2 46 Block diagram of the proposed DAQ system. The digitizing boards are housed in 5 VME crates, each controlled and read by a dedicated Linux PC that uses a VME-PCI interface to access the VME bus. All computers are connected via a network (normal 100 Mbit ethernet or fast optical link if required) to a set of consoles that provide graphical user interface and event display. A custom designed software runs on allcomputers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 47 Basedesignofthedigitizingboard. Inthis"minimal"versiontheboard simplyhasthefunctiontoconverttheinputsignalintoastreamofwords thatare transferred totheDAQcomputersviaVMEbus. . . . . . . . . 119 48 Optional enhanced design of the digitizing board. In this version the board is equipped with a DSP that can perform additional computing functionslikeeventselection ordigitalfiltering. . . . . . . . . . . . . . 120 49 GanttdiagramillustratingtheCUORE detectorfabricationschedule. . . 132 50 GanttdiagramillustratingtheCUORE project timeschedule. . . . . . . 135 51 DetailsoftheCUOREsinglemodulesaccountedforintheMonteCarlo simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141