Universita` degli studi di Genova Facolta` di scienze Matematiche, Fisiche e Naturali Scuola di Dottorato in Scienze e Tecnologie per l’informazione e la conoscenza XX Ciclo, A.A. 2005 - 2008 A Thesis submitted for the degree of Dottore di Ricerca in Fisica Nucleare e Subnucleare Characterization of cryogenic bolometers and data acquisition system for the CUORE experiment School Coordinator Prof. Pierantonio Zangh`ı Universit`ae sezione INFN di Genova Author Dott. Andrea Giachero Advisors Universit`adi Genova e Prof. Marco Pallavicini Laboratori Nazionali del Gran Sasso Universit`ae sezione INFN di Genova Dott. Carlo Bucci Laboratori Nazionali del Gran Sasso Area 02 - Scienze Fisiche Settore Scientifico-Disciplinare: FIS/04 Contents Introduction 2 Part I - Theoretical Overview 3 1 The massive neutrino 5 1.1 Introduction................................. 5 1.2 MajoranaandDiracNeutrino . 7 1.3 NeutrinoOscillations . 9 1.4 Neutrinomasshierarchy . 12 2 Double Beta Decay 15 2.1 Introduction................................. 15 2.2 Possibledecays ............................... 16 2.3 DecayrateandMajoranaMass . 22 2.4 Nuclearmatrixelements . 26 2.5 Experimentaloverview . 27 2.5.1 Past and existing experiments . 31 2.5.2 Futureexperiments . 34 3 Bolometric Technique 39 3.1 Introduction................................. 39 3.2 Generalprinciples.............................. 40 3.3 Theenergyabsorber ............................ 42 3.3.1 Thermalization processes . 44 3.3.2 EnergyResolution . 46 3.4 Thephononsensor ............................. 48 3.4.1 SemiconductorThermistor . 48 3.4.2 Transitionedgesensors. 50 3.5 Detectoroperation ............................. 50 3.5.1 Signalamplitude . 52 3.5.2 Detectornoise............................ 54 Part II - Tellurium Double Beta Decay 57 II CONTENTS 4 CUORE experiment 59 4.1 Introduction................................. 59 4.2 Thelocation................................. 61 4.3 CUORE bolometers ............................. 62 4.3.1 The Double Beta Decay source: 130Te............... 62 4.3.2 The energy absorber: TeO2 .................... 65 4.3.3 Thesensor.............................. 65 4.4 Themodularstructure . 70 4.4.1 Thesinglemodule ......................... 70 4.4.2 Fromthesupermoduletotheentiredetector . 73 4.5 Thecryogenicssetup ............................ 74 4.5.1 Shieldingrequirements . 76 4.6 TheHut................................... 78 4.7 CUORE performances............................ 80 4.7.1 Background interpretation . 80 4.7.2 Doublebetadecayprospects. 81 5 The pilot experiment: CUORICINO 83 5.1 Introduction................................. 83 5.2 Neutrinolessdoublebetadecayresults . 84 5.3 Backgroundanalysis ............................ 85 5.4 Backgroundinterpretation . 88 5.5 Motivation for further R&D activity . 90 6 CUORE background reducing program 93 6.1 Introduction................................. 93 6.2 TheHallCR&Dfacility .......................... 94 6.3 The RAD detector.............................. 95 6.3.1 The RAD1 run ........................... 98 6.3.2 The RAD2 run ........................... 101 6.3.3 The RAD3 and RAD4 runs..................... 103 6.3.4 RAD5 and RAD6 runs........................ 105 6.3.5 The RAD conclusion ........................ 108 6.4 The CAW detector ............................. 108 6.4.1 Development of surface sensitive elements . 110 6.4.2 The CAW1 run ........................... 111 6.4.3 The CAW2 run ........................... 115 6.4.4 The CCT1 run............................ 118 6.4.5 The CCT2 run............................ 122 6.4.5.1 Detector performances . 123 6.4.5.2 Detector background . 124 6.4.6 The CCT conclusion ........................ 125 CONTENTS III Part III - Technical Aspects 133 7 CUORICINO/CUORE electronics 135 7.1 Introduction................................. 135 7.2 Front-EndBoard .............................. 137 7.2.1 The load resistor and the biasing systems . 137 7.2.2 Thepre-amplifier . 138 7.2.3 The programmable gain amplifier . 142 7.2.4 Thedigitalcontrolboard. 143 7.2.5 Thewiring.............................. 143 7.2.6 The cold electronics . 144 7.3 BesselBoard................................. 145 7.4 PulserBoard ................................ 147 8 CUORE Data Acquisition System 153 8.1 Introduction................................. 153 8.2 DAQrequirements ............................. 154 8.3 Apollo: thearchitectureofthesystem . 157 8.3.1 Thehardwareconfiguration . 157 8.3.2 The data acquisition and control software . 158 8.4 TheHallCsetup .............................. 162 8.4.1 RAD5 DAQ test........................... 164 8.4.2 CCT1 DAQ test........................... 165 8.4.3 CCT2 DAQ test........................... 167 Conclusion 173 Appendix 175 A Cryogenics 175 A.1 Dilutionrefrigerator . 175 A.1.1 Properties of 3He/4Hemixture................... 175 A.1.2 Thecoolingsystem ......................... 177 A.2 PulseTubeCryocooler . 179 B Thermistors and electronics 181 B.1 Thermistor Logarithmic Sensitivity . 181 B.2 ThermistorSignalAmplitude . 182 List of figures 185 References 205 Introduction Our knowledge of the neutrino properties has grown steadily and significantly in the last ten years. The existence of the neutrino oscillations, discovered in 1998 by Super- KamiokaNDE in the atmospheric neutrino spectrum and confirmed in 2001 by the SNO experiment in the solar neutrino spectrum, proves that at least two out of three neu- trino flavors have a non zero mass and that lepton number is not a good quantum number. Nevertheless, while precision measurements of the oscillation parameters are planned for the near future, there are still two important missing pieces in the neutrino puzzle: the understanding of the nature of the mass term (Dirac or Majorana) and the measurement of the absolute mass scale. In fact, neutrino oscillations are sensitive only to the squared mass differences between neutrino flavors, yielding no information at all concerning the absolute mass scale. Besides, the oscillation process does not depend on the Majorana or Dirac nature of the mass terms. Neutrinoless double beta decay has long been recognized as a useful avenue for the study of electron neutrinos properties, and particularly for the measurement of the absolute mass scale and for the determination of the nature of the electron neutrino effective mass. In fact, the detection of such a very rare nuclear decay mode would set a lower limit on the mass of the electron neutrino, as well as prove that the neutrino is a Majorana particle. Moreover, characteristics of this mode such as lepton number violation have repercussions that constitute evidence for physics beyond the Standard Model, and impact cosmology as well. The data obtained by the neutrino oscillation experiments put limits on possible value of neutrino masses. There are three possible scenarios: the so called quasi degen- erate scenario foresees three almost equal values of the neutrino masses below but not much below 1 eV; the so called inverted hierarchy and the direct hierarchy scenarios allow a much lighter electron neutrino mass, of the order of 10 meV and 1 meV respec- tively. The quasi degenerate scenario is being investigated right now by the current generation of double beta decay using different and complementary techniques with various nuclei. A new generation of detectors is now rising up with the claim to look inside the inverted hierarchy mass region. The bolometric detectors, which this thesis is all about, are based on tellurium dioxide (TeO2) crystals and play a leading role in this new generation of experiments. This Ph.D. work has been performed in the framework of the CUORE (Cryogenic Underground Observatory for Rare Events) experiment, a tellurium dioxide array of 988 2 Introduction detectors with the aim to search the double beta decay. The expected sensitivity on the neutrino mass is supposed to be better than 50 meV. A prototype of CUORE, CUORI- CINO, is already running at the Gran Sasso National Laboratory of INFN (Istituto Nazionale di Fisica Nucleare): the data it collected in the last five years demonstrated the feasibility of CUOREsetting, at the same time, the best current upper limit for the 130-tellurium neutrinoless double beta decay half-life During the past three years, two of which entirely spent at the Gran Sasso National Laboratory, I have worked in the CUORE collaboration dealing with different aspects of the experiment. My research activity was focused manly on two tasks: the R&D activ- ity aimed at the reduction of the detector background, and the design and development of the data acquisition system for the CUORE experiment. The background reduction is the most important task in view of CUORE. Back- ground is the only parameter that can be reduced by orders of magnitude thus allow- ing a sizeable improvement of the experimental sensitivity. The CUORE background reduction program foresees a reduction from the present CUORICINO level of 0.18 counts/keV/kg/years to about 0.01 counts/keV/kg/years. The performed tests during my Ph.D work was devoted to a reduction of the surface radioactive contaminations of the detector main components: the copper holders and the tellurium dioxide crys- tals. These contaminations, due to the alpha particle contributions, are believed to be at the origin of the flat continuous background in the neutrinoless double beta decay region. The results of these tests showed a relevant reduction for the crystal surface contribution. Unfortunately, only a smaller improvement was reached for the copper