Abstract
Neutrino mass is the sole experimental evidence of physics beyond the Standard Model of particle physics. The quest for understanding the mecha- nism by which neutrinos gain mass drives the search for neutrinoless double beta (0νββ) decay, a process that would indicate a Majorana mass term as the origin for the tiny neutrino masses. The potential of 0νββ decay to distinguish between normal and inverted neutrino mass orderings further highlights its significance and emphasizes the necessity of accurately calculating the nuclear matrix elements (NMEs) necessary to translate the experimental observable of the decay half-life (T0ν ) to the effective Majorana mass (m ). 1/2 ββ In this context, this thesis presents advancements in 0νββ-decay search with Gerda and Legend, alongside ordinary muon capture (OMC) measure- ments conducted by Monument, intended to reduce uncertainties in NME calculations for 0νββ decay. Gerda, a pioneering experiment in probing 0νββ decay of 76Ge with high- purity germanium (HPGe) detectors, concluded its Phase II data taking after accumulating over 100 kg.yr of data. Operating in liquid argon (LAr), Gerda achieved the lowest background index in the field, setting stringent constraints 0ν on T1/2. In this work, the characterization of background rates during Phase II was conducted, providing essential information regarding its background uniformity. Building upon Gerda, Legend-200 similarly employs arrays of HPGe de- tectors immersed in LAr and surrounded by an instrumentation system that collects LAr scintillation light. This LAr system proved itself very effective in vetoeing background events in Gerda, playing a major role in obtaining the remarkably low background index. To facilitate the collection and detection of scintillation light resulting from radioactive decays in LAr, wavelength shift- ing materials such as tetraphenyl butadiene (TPB) are employed to convert 128 nm scintillation light into visible wavelengths. While Gerda also incorporated wavelength shifters, Legend-200 introduced a novel feature surrounding the HPGe detector arrays: a wavelength-shifting reflector (WLSR) composed of Tetratex® (TTX) reflective film coated with TPB. Designed with the primary goal of enhancing light collection efficiency, this WLSR contributes to improving background suppression achieved with the LAr instrumentation. Crucial efforts in attaining this enhancement compose a central part of this work, which included the WLSR design followed by the successful coating of 13 m2 of TTX with TPB. A sample extracted from the resulting WLSR was characterized with microscopy and in LAr. The coating demonstrated satis- factory uniformity, with an estimated quantum efficiency of approximately 85% at LAr temperatures. For even larger surfaces, the complexity of TPB coating presents a challenge, prompting the need for scalable alternatives, such as the plastic wavelength shifter polyethylene naphthalate (PEN). This alternative is particularly relevant for the forthcoming Legend-1000 experiment, which aims to escalate the deployment of HPGe detectors from 200kg (as in Legend-200) to 1000kg, necessitating extensive WLSR coverage. While the optimization of PEN-based WLSR is still an ongoing effort, this work demonstrates its potential and identifies opportunities for enhancing its light yield, including the reduction of effective light absorption within the PEN-based WLSR combinations and tackling challenges posed by the installation of thin films. Within the context of the Monument experiment, OMC was measured in 76Se and 136Ba, with the goal of providing OMC rates for refining the calculations of NMEs associated with the 0νββ decay of 76Ge and 136Xe – the leading isotopes in the field. This work primarily focused on analyzing OMC in 136Ba, identifying over one hundred γ and x-ray emission lines in the measured energy spectrum, thereby laying the foundation for obtaining the OMC rates required to refine the relevant NME calculations. Beyond the scope of 0νββ decay research, this thesis explored the potential of crystals read out by light-sheet fluorescence microscopy (LSFM) as passive particle detectors. This work was performed in the context of Paleoccene, which is based on the utilization of color centers (CC) as potential indicators of nuclear recoils induced by dark matter and coherent elastic neutrino-nucleus scattering (CEvNS). This thesis presents the first measurements utilizing state- of-the-art LSFM microscopes to read out radiation-induced CCs in transparent crystals, demonstrating the feasibility of CC imaging with LSFM, a pivotal component of the Paleoccene concept.
Rodrigues Araujo, Gabriela