Background analysis of the experiments MiDBD, CUORICINO and CUORE
2017
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.
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