17 Décembre – Thesis defense - Miroslav Macko

14 h Seminar rooom - CENBG (Gradignan)

SuperNEMO Experiment: Study of Systematic Uncertainties of Track Reconstruction and Energy Calibration. Evaluation of Sensitivity to 0nubb with Emission of Majoron for Se-82.

Presented thesis is composed of variety of projects which I performed within the construction phase of SuperNEMO demonstrator during the period 2015-2018.
SuperNEMO experiment, located at underground laboratory LSM, is designed to search for 0nubb of Se-82. Its technology, which takes advantage of particle tracking, is unique in the field of double beta decay experiments. Event topology reconstruction is powerful tool for suppression of naturally-occurring background radiation.
Part of the thesis is dedicated to experimental work. I took part in assembly and testing of optical modules - the integral part of SuperNEMO calorimeter. Results of tests after assembly of 520 optical modules are presented in the thesis. Furthermore, I present results of complete mapping of Bi-207 sources performed using pixel detectors. I also present precise measurements of their activities for which I used HPGe detectors. These Bi-207 sources will be used for calibration of the calorimeter. Study played a key role in choice of 42 sources which were installed in the demonstrator and will take part in calibration of the demonstrator.
Another part of the thesis contains projects focused on Monte Carlo simulations. In first of them, I studied a vertex reconstruction precision achievable by reconstruction algorithm developed for SuperNEMO experiment. Precision is evaluated using different statistical methods in variety of different conditions (magnetic field, energy of electrons, angles of emission, etc.). Factors influencing the precision, based on the achieved results are discussed.
In 2018, I also performed simulations of neutron shielding. Variety of shielding materials with different thicknesses were (in the simulation) exposed to realistic neutron spectrum from LSM and the fluxes behind the shielding were estimated. It was shown that the parts of the detector made of Iron should be expected to capture vast majority of neutrons passing the shielding. I also discuss a problem with simulation of deexcitation gamma radiation, emitted after thermal neutron capture, which arises in standard software packages. I proposed new extended generator capable to resolve the problem and demonstrate the concept in analytically solvable example.
Along with standard 0nubb, SuperNEMO will be capable of searching for more exotic modes of the decay. In the thesis, I present possible half-life limits achievable by SuperNEMO for 0nubb with emission of one or two Majorons. The study is performed as a function of activity of internal contamination from Tl-208 and Bi-214 isotopes. Measurement period after which SuperNEMO should be able to improve half-life limits of NEMO-3 (in case the decay would not be observed) are estimated.

Event localization