Model of the Response Function of CUORE Bolometers

The neutrino is probably the most elusive elementary particle discovered so far. Its mass is very small and still unknown, and it is considered a key quantity in many theories beyond the Standard Model of particle physics. The smallness of the mass could be explained if neutrinos are, unlike all other particles, equal to their own antiparticles, thus following the conjecture of E. Majorana.

The double beta decay without emission of neutrinos is a nuclear process that can happen only if the neutrino is a Majorana particle. Observation of this decay would therefore necessarily imply that neutrinos are Majorana particles and would set the mass scale, a breakthrough in our understanding of nature.

The CUORE experiment will search for the neutrinoless double beta decay in 130Te, using 1 ton of TeO2 bolometric detectors. Bolometers are calorimeters that operate at cryogenic temperatures, able to measure the temperature rise produced by the energy release of an impinging particle. They feature good resolution and low background, making them excellent detectors to search for rare decays.

The performances of the experiment are currently limited by temperature instabilities of the bolometers and by a poor understanding of their data. Mea­suring the energy deposited by a particle, in fact, is complicated and the shape of the signal depends on the energy.

In this thesis a model of the signal of TeO2 bolometers is developed. It is able to explain the origin of the unwanted features that would limit the performances of CUORE. The application of the model to data from test bolometers led to great improvements of the results in terms of energy resolution, energy calibration, and signal shape discrimination.

This thesis has been awarded at the Department of Physics, Universita di Roma - La Sapienza, Italy. With a Foreword by Professor Fernando Ferroni.

Auteur
Marco Vignati received his Master Degree in Physics from the Università di Roma - La Sapienza in June 2004 with final mark 110/110 cum laude. His degree thesis was entitled CP asymmetry measurement within bs transitions with the BaBar experiment and his supervisors were Prof. Fernando Ferroni and Dr. Gianluca Cavoto. In January 2010 he received his Ph.D. in Physics from the same University under the supervision of Prof. Fernando Ferroni.

Résumé

Large mass bolometers are used in particle physics experiments to search for rare processes, like neutrinoless double beta decay and dark matter interactions.
In the next years the CUORE experiment (a 1 Ton detector composed by 1000 crystals of TeO2 operated as bolometers in a large cryostat at 10mK) will be the particle physics experiment with the highest chance of discovering the Majorana neutrino, a long standing and yet fundamental question of particle physics.
The study presented in this book was performed on the bolometers of the CUORE experiment. The response function of these detectors is not linear in the energy range of interest, and it changes with the operating temperature, worsening the performances. The nonlinearity appeared to be dominated by the thermistor and the biasing circuit used to read the bolometer, and was modeled using few measurable parameters.
A method to obtain a linear response is the result of this work. It allows a great improvement of the detector operation and data analysis.
With a foreword by Fernando Ferroni.

Contenu

Foreword by Fernando Ferroni
1 Neutrino masses and double beta decay.- 2 TeO2 bolometric detectors for 0_DBD search.- 3 Model of the response function of CUORE bolometers.- 4 Thermal response analysis.- 5 Thermal response analysis on the Three Towers detector.- 6. Conclusions.
Appendix A: Thermal response analysis on the CCVR detector
Appendix B: Precision measurements on the Three Towers detector.