UNIVERSITA` DEGLI STUDI DELL’INSUBRIA Facolta` di Scienze, Como - Dipartimento di Fisica e Matematica DOTTORATO DI RICERCA IN FISICA E ASTROFISICA XXII CICLO OPTIMIZATION OF THE SINGLE MODULE OF DETECTION FOR THE CUORE-0 EXPERIMENT THESIS SUBMITTED BY Chiara Salvioni A doctoral dissertation in partial fullfillment of the requirements to obtain the Degree of Doctor Philosophiae in Physics June 2010 Supervisor: Prof. Andrea Giuliani Program Coordinator: Prof. Philip G. Ratcliffe To things that happen and to what we say about them, remembering that “Capacity to terminate is a specific grace” -Emily Dickinson Contents Introduction 1 1 Neutrinoless Double Beta Decay in theory and practice 3 1.1 Introducing the neutrino puzzle ........................ 4 1.1.1 A theory with neutrino masses .................... 5 1.1.2 Oscillating flavours ........................... 7 1.2 0ν-DBD: the needle in the haystack ..................... 11 1.2.1 Current status of 0ν-DBD search ................... 16 1.3 The bolometric technique as an instrument for 0ν-DBD search ...... 17 1.3.1 Principles of operation ......................... 19 1.3.2 The crystal absorber .......................... 20 1.3.3 The temperature sensor ........................ 21 1.4 Modeling the TeO2 macro-bolometer ..................... 25 1.4.1 Static behaviour ............................ 25 1.4.2 Dynamic behaviour ........................... 27 1.4.3 Energy resolution ............................ 30 1.4.4 Noise sources ............................. 30 1.5 The Cuoricino experiment and its results ................... 31 1.6 Upcoming developments of 0ν-DBD search ................. 35 1.6.1 The near and the next future: CUORE-0 and CUORE ....... 36 2 Overcoming the Single Module of the Cuoricino detector 39 2.1 The Single Module of the Cuoricino detector ................. 39 2.1.1 The TeO2 crystal absorber ...................... 39 2.1.1.1 The choice of the nuclide: 130Te .............. 40 2.1.1.2 The choice of the material: TeO2 .............. 42 2.1.2 The NTD Ge thermistor ........................ 43 2.1.3 The support structure ......................... 45 2.1.3.1 The heat bath ........................ 45 2.1.3.2 The mechanical holders ................... 45 2.1.4 The Si heater .............................. 46 2.1.5 Other sensor couplings ........................ 48 2.1.5.1 The sensor-to-crystal coupling ............... 48 2.1.5.2 Electric connections ..................... 48 2.1.6 Composite modeling: application to the Cuoricino detector .... 49 2.2 Running a large-mass bolometric experiment ................ 50 2.3 Beyond the Cuoricino single module detector ................ 52 2.3.1 The how-to of sensitivity improvement ................ 52 2.3.1.1 Detector mass ........................ 54 2.3.1.2 Energy resolution ...................... 54 2.3.1.3 Measurement time and duty cicle ............. 55 2.3.1.4 Background level ...................... 55 Contents 2.3.1.5 Isotopic enrichment ..................... 57 2.3.2 Towards uniform and optimal bolometric performances ....... 59 2.3.2.1 Pulse shape and response ................. 59 2.3.2.2 Energy resolution ...................... 60 2.3.2.3 Detector modularity ..................... 60 2.4 Designing a CUORE-oriented specific R&D activity ............. 60 2.4.1 CUORE-0 as CUORE’s test bed ................... 63 3 The chase for background reduction 65 3.1 The identification of relevant background sources .............. 65 3.1.1 An investigation on the Cuoricino background ............ 65 3.1.2 Past R&D activity on background issues ............... 67 3.2 Getting to grips with the radioactive background ............... 70 3.3 Inert detector elements and surface contaminations: active approach ... 72 3.3.1 Discrimination of surface events with TeO2 macro-bolometers ... 73 3.3.2 Potentialities of SSBs in the search for the 0ν-DBD of 130Te .... 74 3.4 Inert detector elements and surface contaminations: passive approach .. 77 3.4.1 Surface treatment of copper elements ................ 77 3.4.1.1 The classic RAD-Polyethylene protocol .......... 78 3.4.1.2 The new LNGS protocol ................... 79 3.4.1.3 The Legnaro protocol .................... 80 3.4.2 The Three Towers Test ........................ 82 4 A close look at the crystal absorbers 87 4.1 Crystal absorbers: what to work on ...................... 87 4.2 Production and processing of TeO2 crystals ................. 91 4.3 Testing the radio-purity of TeO2 absorbers .................. 92 4.3.1 The first Chinese Crystals Test .................... 95 4.3.2 The second Chinese Crystals Test .................. 100 4.3.3 The ultimate production protocol ................... 105 4.4 An investigation on crystal producers ..................... 106 4.4.1 Sources of TeO2 crystals: a dedicated test ............. 107 4.5 Final remarks .................................. 116 5 Innovative contact geometry for germanium thermistors 117 5.1 A matter of assembly .............................. 117 5.2 Tampering with contact geometry: dos and don’ts .............. 120 5.2.1 General approach to thermistor design ................ 120 5.2.1.1 Thermal properties ..................... 121 5.2.1.2 Electric properties ...................... 121 5.2.1.3 Pad size ........................... 122 5.2.1.4 Thermistor “footprint” .................... 122 5.2.2 Geometry effects on the internal electric field ............ 123 5.2.3 Remarks on comparative testing ................... 125 5.3 Notched thermistors .............................. 132 5.4 Flat-pack thermistors with small geometry .................. 133 5.4.1 The how-to of flat-pack thermistors .................. 133 5.4.2 Test 1: introducing small flat-packs .................. 136 5.4.3 Test 2: small flat-packs and large mass detectors .......... 142 5.5 Flat-pack thermistors with large geometry .................. 145 5.5.1 Test 3: elimination round in Como .................. 148 5.5.2 Test 4: large flat-packs at LNGS, first round ............. 152 5.5.3 Test 5: large flat-packs at LNGS, second round ........... 157 5.6 Wrap-around thermistors ........................... 157 ii 5.6.1 Test 6: meet the wrap-arounds .................... 161 5.6.2 Test 7: final run at LNGS ....................... 165 5.7 Drawing conclusions: the choice for CUORE-0 ............... 166 6 The sensor to absorber coupling 177 6.1 A soft point in the bolometric model ...................... 177 6.2 Coupling sensors to crystals: restrains to the process ........... 183 6.2.1 The “Araldite affaire” .......................... 183 6.2.2 Defining the geometry of the epoxy interface ............ 186 6.2.3 Restrains on environmental conditions ................ 187 6.3 A brief history of gluing ............................ 189 6.4 The CUORE-0 way to gluing ......................... 191 6.4.1 Epoxy mixing .............................. 192 6.4.2 Epoxy dispensing ........................... 194 6.4.2.1 The air-free epoxy dispenser ................ 199 6.4.2.2 The pneumatic epoxy dispenser .............. 202 6.4.2.3 Comparison between dispensing techniques ....... 203 6.4.3 Sensor positioning ........................... 205 6.4.4 Introduction of automated elements ................. 209 6.4.5 A final glimpse of the gluing line ................... 210 6.5 Future steps .................................. 211 Conclusion 215 A Cryogenic systems 217 B Signal read-out 223 C First-level data analysis 225 Glossary 229 List of Figures 233 List of Tables 236 Bibliography 240 Acknowledgements 241 iii Introduction Neutrino physics is nowadays one of the most discussed research fields in the study of the fundamental constituents of matter and their interactions. Its popularity has been in- creasing in the last years, supported by the wealth of experimental facts that have given model-independent proof of neutrino oscillations: the observation of oscillating neutri- nos, first discovered by SuperKamiokande, then confirmed by SNO, has contributed significantly in pushing the efforts of the physics community towards a revision of the Standard Model. The fact that neutrinos oscillate between flavours can be understood only if a mixing mechanism, similarly to what happens in the hadronic field, is introduced, which leads directly to the request that these particles are admitted to be massive; and, for the Standard Model to incorporate non-zero neutrino masses, a certain number of its foundations should be relaxed. Among them, the redefinition of neutrinos as equal to their own anti-particles, which is a condition defined as “Majorana nature”, is particularly appealing to theorists. Experiments on flavour oscillations by neutrinos will shortly follow the path of preci- sion measurements. This perspective, however, although needed, does not lead to any radical clarification about the topics that are still open today: a measurement on oscillat- ing neutrinos does not quantify their mass eigenvalues but only the difference between their squares, nor it is capable of determining which nature, Majorana or Dirac, is valid for them. The values of squared mass differences obtained by these experiments are compatible with three possible scenarios: one where all masses are of order ∼1 eV and very similar to each other, and two that require one mass to be much smaller (inverse hierarchy) or larger (normal hierarchy) than the other ones. It is true that the absolute values of neutrino mass eigenvalues could be determined by cosmological measure- ments or by end-point study in single β-decay, but the first is model-dependent and the second one cannot