UNIVERSITA` DEGLI STUDI DI MILANO - BICOCCA FACOLTA` DI SCIENZE MATEMATICHE, FISICHE E NATURALI CORSO DI DOTTORATO DI RICERCA IN FISICA E ASTRONOMIA XXIII CICLO, A.A. 2008 { 2010 ATHESIS SUBMITTED FOR THE DEGREE OF Dottore di Ricerca in Fisica Development of cryogenic detectors for rare event searches Luca Gironi Matricola 033675 Advisor School Coordinator Prof. Oliviero Cremonesi Prof. Claudio Destri Contents Contents 1 Introduction1 PART I: Theoretical Overview5 1 The massive neutrino7 1.1 Neutrino history.................................7 1.2 Neutrino oscillations...............................8 1.3 Absolute measurement of neutrino mass.................... 10 1.4 Dirac and Majorana neutrinos.......................... 11 2 Neutrinoless Double Beta Decay 13 2.1 Double Beta Decay................................ 13 2.1.1 Nuclear Factor of Merit......................... 16 2.1.2 Experimental signature......................... 18 2.2 Experimental Techniques............................ 19 2.2.1 Status of the Art............................. 20 2.2.2 Future experiment............................ 22 3 0νDBD sensitivity and background 26 3.1 Experimental sensitivity............................. 26 3.2 Energy resolution................................. 28 3.3 Detector material................................. 29 3.4 Radioactive background in macro-bolometer.................. 31 3.4.1 Internal background........................... 31 3.4.2 External background sources...................... 32 3.4.3 Surface contamination.......................... 36 4 The Bolometric Technique 39 4.1 Working Principles................................ 39 I Contents 4.2 Energy Absorber................................. 42 4.3 Thermalization process.............................. 43 4.4 The phonon sensor................................ 45 4.4.1 Semiconductor thermistor........................ 45 4.4.2 Transition Edge Sensor......................... 46 4.5 Detector operation................................ 47 4.6 Noise........................................ 48 4.7 Read-out, DAQ and analysis technique..................... 50 PART II: TeO2 0νDBD 53 5 Cuoricino 55 5.1 Experimental Setup............................... 56 5.2 Cuoricino TeO2 bolometers........................... 58 5.3 Cuoricino data.................................. 59 5.4 Cuoricino background.............................. 60 5.5 Muon Veto.................................... 63 5.5.1 Muon Detector Setup.......................... 63 5.5.2 Detector Operation and Performance.................. 64 5.5.3 Data Analysis............................... 65 5.5.4 Simulation................................ 68 5.5.5 Simulation Results............................ 69 6 CUORE 72 6.1 The detector................................... 72 6.2 Cryostat and shieldings............................. 74 6.3 Geant4 simulation............................... 75 6.3.1 Simulation results............................ 77 6.4 CUORE sensitivity................................ 81 7 Research and Development 84 7.1 Experimental setup and physical goals..................... 84 7.2 The Three Towers Test.............................. 87 PART III: Scintillating Bolometers 91 8 Scintillating Bolometer 93 8.1 Working principles................................ 93 8.2 Experimental Setup............................... 95 8.2.1 The light detector............................ 98 8.3 Scintillating bolometer for 0νDBD....................... 99 II Contents 8.3.1 CdWO4 .................................. 99 8.3.2 ZnSe.................................... 113 8.3.3 Molybdates................................ 122 8.3.4 Summary of 0νDBD scintillating bolometer.............. 127 8.4 Surface contaminations studies......................... 129 8.4.1 The choice of the scintillating crystal.................. 130 8.4.2 The array of 2×2×2 cm3 BGO crystals................ 132 8.4.3 The 5×5×5 cm3 crystal......................... 140 9 Pulse Shape Analysis in bolometer 145 9.1 PSA advantages................................. 145 9.2 Pulse shape signature in the heat pulse..................... 146 9.3 CaMoO4 ...................................... 147 9.4 ZnMoO4 ...................................... 150 9.5 MgMoO4 ..................................... 151 9.6 ZnSe........................................ 151 9.7 The energy partition and the PSA....................... 154 10 0νDBD experiments with scintillating bolometers 156 10.1 A strategy for the future: bolometers...................... 156 10.1.1 The background............................. 158 10.1.2 Near sources............................... 159 10.1.3 Far sources................................ 162 10.1.4 Environmental background....................... 162 10.1.5 Energy resolution............................. 163 10.1.6 Mass.................................... 164 11 Conclusion 167 Appendix 169 Bibliography 172 III To Magda Introduction The experimental evidence of neutrino oscillations clearly showed that the neutrino is a finite-mass particle. Anyway, two big questions concerning the neutrino are still unsolved: its nature (Dirac or Majorana) and the absolute value of its mass. The Double Beta Decay without emission of neutrinos (0νDBD) is at present the most sensitive method to answer the two questions. Bolometers, together with germanium diodes, have provided so far the best results within this kind of research. The choice of the so-called calorimetric approach, where the detector is made of the 0νDBD material, allows the study of large quantities of isotope with an excellent energy resolution (around 0.2-0.3 %). Both features are crucial to disentangle the searched peak from background. The Cuoricino experiment, in which 62 detectors of tellurium dioxide (TeO2) were involved, was carried out in hall A at Labo- ratori Nazionali del Gran Sasso (LNGS), and represented the bolometric experiment with the best sensitivity concerning the study on the 0νDBD decay. The knowledge achieved in the bolometric technique and its excellent results on 0νDBD led to the development of new experiments to study rare events with bolometric technique. The CUORE (Cryogenic Underground Observatory for Rare Events) experiment, composed by 988 bolometric de- tectors of TeO2, is under construction and foresees a sensitivity on neutrino mass of the order of about 50 meV. This high sensitivity requires excellent energetic resolutions, low number of spurious counts within the region of interest and high quantity of the 0νDBD active isotope. The reduction of the background in the energetic window where the 0νDBD 130 for the isotope Te is expected (Qββ = 2527 keV) plays a primary role. In this context, the activities I have done are focused primarily on the analysis of the different contributions to the background of CUORE and the optimization of methods for its reduction. This work has been done both through measurements performed in the hall C of LNGS and with Monte Carlo simulations that allowed data interpretation and assessment of the resulting sensitivity. For example, through a series of simulations performed with Geant4 code it is possible to extrapolate the background induced by muons in CUORE. The results of simulations were then validated through the installation of a muon veto above Cuoricino which allowed a direct comparison with experimental measurement. For what concerns the reduction of the bolometers specific background I have also done some measurements for the development, characterization and optimization of some scintillating bolometers. These detectors, thanks to double read-out of thermal and scin- tillation signals, allow to discriminate different ionizing particles (β/γ, α and neutrons). This allows a significant reduction of the unwanted background in the region of interest and would therefore allow to approach the condition of `zero background' experiments. In- deed, thanks to the wide choice for the absorber material that allows to study practically all 0νDBD candidate isotopes, it is possible to choose an isotope with a transition energy above 2615 keV and then remove, in practice, all the background due to γ rays. The 2615 keV γ line corresponds in fact to the highest energy γ-ray line from natural radioac- tivity and is due to 208Tl. Above this energy there are only extremely rare high energy γ's. Once γ-rays are no more a worrisome source of background, what is left on the side of radioactivity are α emissions. Indeed α surface contaminations are already recognized as the most relevant background source in the bolometric experiment Cuoricino. However, thanks to the double read-out of scintillating bolometers, this source of background can be removed too. During my PhD I have tested a number of different crystals and with some of them (CdWO4, ZnSe and some molybdates) I have obtained excellent results both from the point of view of the detector performance and the reduction of the background contributions. Thanks to these results it was possible to evaluate the discrimination power (i.e. the capability to recognize and reject unwanted events) of this technique and extrap- olate the sensitivity of a large mass experiment for 0νDBD based on the hybrid heat plus scintillation technique. Moreover, during the analysis of the data collected with the scintillating bolometers, I recognized for the first time, a dependence of the pulse shapes (both on the scintillation and heat channels) on the interacting particle nature. I proposed then to exploit such wonderful feature to discriminate the interacting particles without