From Legend-200 background studies to the design of active shielding against cosmogenic neutrons in Legend-1000

The discovery of neutrino oscillations established that neutrinos have non-zero mass, providing clear evidence of physics beyond the Standard Model. Yet, fundamental questions remain: the absolute neutrino mass scale, their ordering, and whether neutrinos are Majorana particles. The observation of neutrinoless double beta decay (0νββ) would demonstrate lepton number violation and confirm the Majorana nature of neutrinos, offering a direct probe of the effective Majorana mass and insight into the origin of neutrino masses. High-purity germanium (HPGe) detectors enriched in Ge76 provide one of the most sensitive technologies for 0νββ searches, combining excellent energy resolution, high detection efficiency, and powerful background rejection. Building on the success of the GERDA experiment, the LEGEND collaboration pursues a staged program to reach nearly background-free conditions and explore half-lives up to 10^28 years. This thesis presents contributions to the LEGEND experimental program, focusing on data quality assessment, background characterization in LEGEND-200, and background reduction strategies for LEGEND-1000. A detailed analysis of γ-line intensities in the LEGEND-200 spectrum was performed to identify and quantify background components. For LEGEND-1000, both passive and active mitigation strategies against cosmogenic backgrounds—particularly those arising from neutron capture on Ge76 producing Ge77 and Ge77m—were investigated. Monte Carlo simulations were used to optimize a passive neutron shield within the liquid argon cryostat, balancing neutron moderation efficiency, radiopurity, and mechanical constraints. In parallel, the design and performance of an active LAr neutron veto were studied through detailed simulations of scintillation light production, photon transport, and photodetector response. The resulting framework enables quantitative evaluation of veto efficiency and informs the design of future large-scale 0νββ experiments.