Background analysis of the experiments MiDBD, CUORICINO and CUORE

During the last few years the neutrino scenario has dramatically changed. Atmospheric neutrino [50], solar neutrino [51] and reactor antineutrino [53] [1] experiments have given model independent evidences of neutrino oscillations. In this scenario there are two general theoretical possibilities for massive neutrinos, depending on the conservation or not of the total lepton charge. In the first case neutrinos are Dirac particles, while in the second they are Majorana particles. At the moment the question about the neutrino nature is still without an answer. The solution of the problem of the nature of the massive neutrinos will be of fundamental importance to understand the origin of the small neutrino masses and of the pattern of neutrino mixing. The investigation of neutrino oscillations does not allow to solve this problem. Experiments looking for the DBD of even-even nuclei have the highest sensitivity to possible violations of the total lepton number L and to Majorana neutrino masses. DBD experiments try to measure the effective Majorana electron neutrino mass by measuring the rate of the DBD transition. Unfortunately the calculations of the nuclear matrix elements presently available [21] [10] [43] [6] show a spread of results, due to different models and hypothesis used for their computation. To overcome this problem it is fundamental to search for DBD on several nuclei [7]. DBD can be searched for with different experimental methods. One possible direct approach is based on the bolometric technique. The energy released in dielectric and diamagnetic crystals gives rise to measurable temperature increases when working at low temperature (T 10 mK). Cryogenic detectors offer therefore a wide choice of DBD candidates. The isotope Te is and excellent candidate to search for DBD due to its high transition energy (2528.8 1.3 keV) and large isotopic abundance (33.8%) which allows a sensitive experiment to be performed with natural tellurium. Of the various compounds of this element, TeO appears to be the most promising, due to its good thermal and mechanical properties. Because of the rarity of the searched process, spurious counts due to environmental radioactivity, intrinsic contaminations and cosmic ray activation of the detector and of the other experimental setup materials, airbone activity (Rn) and neutrons can obscure the signal counts of interest. A good knowledge of the radioactive sources that mainly contribute to the measured background in the DBD energy region, is therefore of fundamental importance in order to study new strategies to reduce such contaminations and consequently improve the sensitivity of the experiment. This PHD thesis work was focused on the analysis of the data collected with two bolometric experiments, aimed to search for the DBD of the isotope Te . It mainly consisted in the development of a background model, able to describe the observed spectra in terms of environmental radioactivity, radioactive bulk contaminations of the whole detector setup and surface contaminations of the material directly facing the detector itself. The developed model is based both on the analysis of the collected data by means of sophisticated analysis procedures and on the direct comparison between measured and Montecarlo simulated spectra. The Montecarlo spectra are obtained by means of a C++ code, based on the Geant4 package. With this code it is possible to simulate detected events due to radioactive contaminations of the environment or of the various experimental parts. Different detector geometries has been introduced in the code, in order to account for the different structures of the analyzed experiments. The first large mass array (MiDBD experiment) to which the present PHD research activity has been devoted, consisted of 20 TeO bolometers of 340 g each (3 3 6 cm crystals), four of which isotopically enriched (two enriched in Te and two in Te with isotopic abundance of 82.3 and 75 respectively), for a total TeO mass of 6.8 kg. It was operated since 1997 in the hall A of the National Laboratories of Gran Sasso (LNGS). From the analysis of the background data of this experiment we gained a better knowledge of the radioactive sources responsible of the measured background, which induced us to develop a radiopure cleaning process for the surfaces of the detectors and of all the experimental parts facing them. The developed cleaning process was adopted in the rebuilding of the MiDBD experiment performed at the end of 2000. The detector was completely rebuilt with a new structure, similar to the one foreseen for the CUORICINO and CUORE experiments, and this allowed to improve the internal roman lead shield. The background spectra measured in both the MiDBD runs were analyzed and by the comparison between them we could verify the effectiveness in reducing the backgroun of the cleaning procedure adopted. The MiDBD experiment was completed in 2001. It was replaced by the larger mass experiment CUORICINO. The CUORICINO detector is a tower-like structure made by eleven planes of 4- crystal modules (5 5 5 cm crystals of 790 g mass each) and two additional planes of 9-crystal modules (3 3 6 cm crystals of 330 g mass each), for a total TeO mass of 40.7 kg. All the crystals are made of natural tellurium but 4 isotopically enriched crystals (the ones already used in MiDBD).With the appropriate new detector geometry, the background model developed for MiDBD was used also to analyze the data acquired with CUORICINO. This same background model, after a correct implementation of the new detector geometry, was used also to evaluate the experimental sensitivity of the next generation experiment CUORE. The CUORE project is a bolometric detector made by about 1000 TeO 5 5 5 cm crystals, arranged in a cylindrical structure of 19 towers. Each tower consistes of 13 planes each, similar to those used in CUORICINO. The designed detector should be able to reach the sensitivity in the range 0.01 eV to the present upper bounds. From the analysis of the background measured with MiDBD-I, MiDBD-II and CUORICINO, important informations have been obtained in terms of materials contaminations and contributions to the background in the DBD energy regions of the performed experiments. These informations have been used to evaluate by means of the Montecarlo code the background level expected in CUORE when using the materials presently at our disposal. At the same time the Montecarlo code has been used to evaluate the maximum contamination level acceptable for every single experimental part in order to reach the required sensitivity. A plan of material selection and radioactive measurements was therefore developed.