This thesis is structured around two main axes: on one hand, my contribution to the NUCLEUS experiment, which aims to detect nuclear recoils (NR) induced by the coherent elastic scattering of reactor neutrinos off nuclei (CEνNS); on the other hand, the development of the CRAB (Calibrated Recoil for Accurate Bolometry) method for the energy calibration of NRs in cryogenic detectors.
The NUCLEUS experiment will soon be installed near the Chooz nuclear power plant (Ardennes, France). At this site, the main sources of background arise from ambient γ-rays, atmospheric neutrons, and cosmic muons. Reducing background is a central challenge for NUCLEUS, as it is essential for observing the CEνNS signal. To address this, the experiment relies on a cryogenic target detector, passive shielding, and two veto systems designed to identify and reject the various background contributions. Prior to its installation at Chooz, the experiment was deployed in an underground laboratory in Munich for a preparation phase, including a blank assembly followed by a First Commissioning Run (FCR). As part of this phase, I was responsible for the commissioning of the muon veto system. I successfully validated the response of all muon veto modules and their performance. I also contributed to the analysis of the FCR data, which demonstrated the proper functioning of the full NUCLEUS detection system. A particular focus was placed on synchronizing the various acquisition systems, to ensure efficient background rejection by the veto systems. In particular, the effectiveness of the muon veto in rejecting cosmic muon-induced backgrounds was clearly demonstrated.
The second part of this thesis focuses on the CRAB method, which I contributed to develop and validate. This method enables calibration of nuclear recoils in the sub-keV energy range, a key step forward for future neutrino and dark matter experiments. The principle is based on the radiative neutron capture on a target nucleus. During the de-excitation via γ emission, the nucleus recoils with a fixed energy determined by two-body kinematics, thus generating a calibration peak in the region of interest. I carried out simulations to validate this method for several target materials (CaWO4, Al2O3, Ge, Si), particularly studying the impact of NR–γ coincidence on the signal-to-background ratio. I also participated in the experimental validation of the method on an Al2O3 target. As part of my joint PhD with the Technical University of Vienna (Austria), I spent one year in Vienna contributing to the instrumental development of Phase II of the CRAB project, carried out at the TRIGA-Mark II research reactor of the Atominstitute, which provides a purely thermal neutron beam. I took part in the commissioning of the γ detectors and was responsible for operating the cryostat. Finally, analysis of the first data collected in Vienna yielded promising results, in good agreement with simulations and confirming the NR–γ coincidence signal.
LEARN Laboratoire d’études et d’applications des réactions nucléaires – Study and Applications of Nuclear Reactions
DPhN, LEARN