Additive Manufacturing of Near Components for Background Reduction and Structural Supports in LEGEND-1000

Detection of 0νββ decay establishes neutrinos as Majorana particles. LEGEND-1000 is the next generation ton-scale effort to detect 0νββ decay in ⁷⁶Ge using isotopically-enriched high-purity (≥ 90%) germanium detectors. This experiment aims for an unprecedented discovery sensitivity of T^1/2 > 10²⁸ yrs, exposure of 10 ton·yr, and background goal of < 10⁻⁵ cts/(keV kg yr). A dominant source of external background arises from ⁴²K β-decay, produced by trace ⁴²Ar in the natural liquid argon cryostat where the detectors are submerged. To shield the HPGe detectors from this background, we study the possibility of custom radiopure encapsulations of individual detectors via additive manufacturing and Monte Carlo simulations. We evaluated select candidate polymers (SLA resins, commercial and synthesized polyethylene naphthalate) and FDM filaments. This entailed optimizing of parameters, printing and performing mechanicial testing of ISO standard dogbones, connector arms, cryostability test pieces, and optical characterization of printed cuvettes and encapsulations. Further efforts were made in filament extrusion and FDM printing with custom filaments and commerical PEN filament. In parallel, we developed a Monte Carlo background study with simulations of radiation transport and ⁴²K decay products using a Julia-based Geant4 wrapper, SolidStateDetectors.jl, to advice the optimal thickness for the encapsulation.