This PhD topic is part of the NUCLEUS experiment, installed at the heart of the Chooz nuclear power plant in France, with an ambitious goal: detecting and measuring coherent elastic neutrino-nucleus scattering (CEvNS) at a nuclear reactor. At MeV energies, CEvNS is the dominant interaction mode of neutrinos with matter, but the very low-energy nuclear recoils it produces long made it unobservable: it was first measured in 2017, forty years after its prediction, at the Oak Ridge accelerator in the USA. Measuring it at a reactor is a twofold challenge: the nuclear recoils, with energies of the order of 100 eV, escape conventional detectors, and background conditions there are particularly unfavorable.
To meet these challenges, the NUCLEUS collaboration has developed cryogenic calorimeters: calcium tungstate crystals with a total mass of about 10 grams, operated at ~10 mK and read out by transition-edge sensors (TES), achieving detection thresholds of the order of 10 eV. These detectors are shielded from external backgrounds by an instrumented cryogenic shield, complemented by a radiological shield and a muon veto outside the cryostat, all designed and developed by our team. These ultra-low thresholds have however revealed a new limitation: a poorly understood instrumental background, dubbed the "low-energy excess" (LEE), whose rate rises exponentially below the keV and inders the expected CEvNS signal. Observed by all low-threshold cryogenic detection experiments, the LEE is today the main factor limiting the cryogenic detection of reactor antineutrinos, and understanding it is the guiding thread of this thesis.
Following a successful installation in 2026, NUCLEUS has entered its first data-taking phase, called the "technical run", using a simplified cryogenic detection module. It will be followed in 2027 by a "neutrino run", exploiting an updated version of the module integrating an instrumented crystal-holding system, called the inner veto, developed at the Technical University of Munich. During the thesis, the PhD student will take part in these data-taking campaigns and their analysis, mobilizing all the sub-detectors of the instrument. The first task will be to characterize particle backgrounds in the keV region in order to refine the Geant4 model developed by our team, an essential prerequisite to properly isolate the temporal and energy characteristics of the LEE in low-energy reconstructed events. A second part of the topic will then be dedicated to the characterization of the LEE and to the identification of its origin among the current hypotheses (mechanical stress induced by the crystal holder, relaxation of crystal defects), through a detailed comparison of the two detector configurations. This comparison will be decisive to test the mechanical stress hypothesis and, where appropriate, to reject the associated events. Combining these two campaigns will make it possible to better identify the experimental parameters that modulate the LEE event rate, and to guide the design of a new detector minimizing this background. The goal is to pave the way for a CEvNS measurement at a reactor with world-class sensitivity, with implications for neutrino physics and the monitoring of nuclear facilities.