Analysis of surface radioactive background contributions and study of rare decays in the cuore experiment

Rare Physics event is playing a crucial role, not only in Fundamental Interaction Physics, but also in Astroparticle Physics and in Cosmology. These signals, if detected, would give an importatnt evidence of new Physics. The CUORE experiment (Cryogenic Underground Observatory for Rare Events) is a proposed tightly packed array of 988 TeO2 bolometers, each being a cube 125 cm3 on a side with a mass of 750 g. The array consists of 19 vertical towers, arranged in a compact cylindrical structure. Each tower will consist of 13 layers of 4 crystals. The design of the detector is optimized for ultralow-background searches. Neutrinoless double-beta decay (ββ0ν) is the main goal of CUORE. What is new is the fact that positive observation of neutrino oscillations gives new motivation for more sensitive searches. Neutrino oscillation experiments can only provide data on the mass differences of the neutrino mass-eigenstates. The absolute scale can only be obtained from direct mass measurements (β-decay end point measurements), or in the case of Majorana neutrinos, more sensitively by neutrinoless double-beta decay observation. ββ0ν is not the only exotic process which can be observed in the CUORE experiment. Other rare events, from cold dark matter, to rare nuclear decays and electron decay can in principle be studied with the CUORE experimental facility. I will discuss the last process in the 6th chapter. The topic which joins the exotic and rare processes discussed is the unwanted radioactive background which is inevitably present in the experimental measurements. CUORICINO, almost a single CUORE tower, was constructed as a smaller scale ex- periment and operated from december 2003 to June 2008. Besides being a sensitive experiment on 130Te double beta decay, CUORICINO is a conclusive test of CUORE. CUORICINO provided important results concerning both the technical performances of the bolometric tower (CUORE will be made of 19 such towers), the background level .In particular, one of the information gained is that the most probable candidates for the continuum background observed in the spectra, are the surface α contaminations of the copper mounting frame. Rare Physics event is playing a crucial role, not only in Fundamental Interaction Physics, but also in Astroparticle Physics and in Cosmology. These signals, if detected, would give an importatnt evidence of new Physics. The CUORE experiment (Cryogenic Underground Observatory for Rare Events) is a proposed tightly packed array of 988 TeO2 bolometers, each being a cube 125 cm3 on a side with a mass of 750 g. The array consists of 19 vertical towers, arranged in a compact cylindrical structure. Each tower will consist of 13 layers of 4 crystals. The design of the detector is optimized for ultralow-background searches. Neutrinoless double-beta decay (ββ0ν) is the main goal of CUORE. What is new is the fact that positive observation of neutrino oscillations gives new motivation for more sensitive searches. Neutrino oscillation experiments can only provide data on the mass differences of the neutrino mass-eigenstates. The absolute scale can only be obtained from direct mass measurements (β-decay end point measurements), or in the case of Majorana neutrinos, more sensitively by neutrinoless double-beta decay observation. ββ0ν is not the only exotic process which can be observed in the CUORE experiment. Other rare events, from cold dark matter, to rare nuclear decays and electron decay can in principle be studied with the CUORE experimental facility. I will discuss the last process in the 6th chapter. The topic which joins the exotic and rare processes discussed is the unwanted radioactive background which is inevitably present in the experimental measurements. CUORICINO, almost a single CUORE tower, was constructed as a smaller scale ex- periment and operated from december 2003 to June 2008. Besides being a sensitive experiment on 130Te double beta decay, CUORICINO is a conclusive test of CUORE. CUORICINO provided important results concerning both the technical performances of the bolometric tower (CUORE will be made of 19 such towers), the background level .In particular, one of the information gained is that the most probable candidates for the continuum background observed in the spectra, are the surface α contaminations of the copper mounting frame. Silicon Barrier Detectors (SBD) are a powerful instrument to study charged particle radiation (like α particles). During my PHD one of the activity I focused on was the optimization of the SBD used in the radioactivity laboratoty of the the University of Milano Bicocca A complete procedure for the calibration of these detectors was set- tled. In fact, one of the main problem to face with, (due also to the extremely low activity measured), is the discrimination of their intrinsic background level from that of the sample measured. The SBD are always operated in Ultra Low Background vacuum chambers. In the context of the discrimination of the background, evaluation of the muon and shower contribution to the acquired spectra were performed. The latter were done through a coincidence measurement between the SBD and a scintillator. The result of the measurements and of their ananlysis showed that the major contribution to the spurious counts comes from the showers. A dedicated acquisition was done for the detectors, with a module which lets to have event’s temporal imformation. To give limits on the surface activities of the samples the use of Monte Carlo (MC) simulation is mandatory in order to have an estimation of the efficiency of energy detection. The use of the MC simulation was optimized: different depht and profiles of contamination were studied and tested. This optimization allows to give limits on surface 232Th, 238U and 210Pb-Po activities which depend on the depht of contamination. The drastic reduction of the sensitivity achieved (from 10−5 to 10−7 and 10−8 Bq · cm2 fo the cleanest material measured) is due to the described optimiza- tion. Last, but non least, the SBD measurements played a crucial role in the material selection, depending on the radiopurity required, for the CUORE experiment. Concering the ββ0ν, a crucial role, in the theoretical interpretation of the experimental result, is played by the Nuclear Matrix Element (NME) used to transalte the observed rate (in the energy region where the signal is expected) in a sensitivity on the effective neutrino mass |mν|. I performed a study in order to compare and understand the different nuclear models used nowadays, and the respective Phase Space Factors (PSF) used. This study allows to compare, in a quantitative way, the different experiments on neutrinoless double beta decay. A database was realized in which all the inputs are collected, comments and references on NME and PSF are illustrated and the kind of short range correlation is used in the calculation of the matrix element. The database, with the information collected and properly organized, allows to evaluate the sensitivity on |mν| of all the experiments now at work, depending on the nuclear model used. The difficulties encountered in the comprehension of the nuclear models and in the PSF used are due to three main reasons. • the PSF shoul be in principle standard and unambiguous, not depending on the nuclear model, but just on the initial and final states JP . This is not what the study showed: the PSF, in the different formulation, show discrepancies of a factor 5 or 6. • the nuclear models assume different approaches to the process and should lead to different results. Two models, i.e., the Shell Model and the IBM (Interactive Boson Model) have a similar approach, but they differ in handling the states which are ’far from closed shells’, so in the handling of the nuclear deformations. The QRPA, (in the version pnQRPA an rQRPA, Quasi Random Phase Approximation), have a different approach to the previous models, because it introduces the concept of quasiparticle, which are states built with a ’mixing’ of creation and annihilation operators (a theory very similar to BCS for the superconductivity) and it leads to a correlation between particles and holes and not just between particles (gph and not only gpp pairing). • the SRC (shot range correlation) used should be univocal, but this is not the case. The theoreticians don’t give a clear choice of the proper SRC to be used. Finally I performed a study on the electron decay, in the channel e− ← γ + ν, using CUORICINO data. Moreover I performed a calculation of the cross section for the process, assuming a massless neutrino in the first step and a massive neutrino in the second step. The study of the channel implies to evaluate the signature of the decay, which depends on the material and atomic shell from which the electron disappears. In fact the visible energy changes if the decay happens in the active volume of the detector or in the surroinding materials: Ev = (mec2−Eb) 2 + EX = (mec2+Eb) 2 where me is the electron mass, Eb is the binding energy, EX is the X-ray energy following the decay. The last term is included only if the decay happens inside the active volume of the detectors. Thus, there are several signatures which can be discriminated from the background only if the detector resolution is excellent. Moreover the doppler broadening of the lines, due to the orbital motion of the electron in the shell, must be considered. I thus studied the different signatures in several materials, (potential emitters). In the analysis I included the efficiences for the signatures, using Monte Carlo simulations, expressely conformed to experimental set-up and charachteristics (such as real thresholds, active channels). The correction to the efficiencies, i.e. the loss of ’good events’, due to the analysis cuts, was evaluated. All the analysis done led to a promising result for this decay, in competition with the current limits given from other collaborations. The cross section calculation allowed to give an estimation of the CNC parameter, using as inputs the available experimental data.