Improving Sensitivities in 0𝒗ββ Decay Searches by Utilizing PEN as a Structural Scintillating Material

Neutrinoless double beta decay, 0nbb is currently the only experimental test to unambiguously determine the majorana nature of the neutrino. There is a large international effort to measure 0nbb decay, with several detector technologies being pursued. This dissertation will consider the LEGEND experiment (Large Enriched Germanium Experiment for Neutrinoless bb Decay), an international effort to measure 0nbb decay with ⁷⁶Ge as both the target isotope and the detecting material.

LEGEND has a 200 kg stage and a 1000 kg stage, each requiring extremely low levels of background radiation at Q_bb (E = 2.039 MeV). These ultra-low background levels are reached partially by active veto systems like the liquid argon (LAr) cryostat, acting as a cryogenic for the ⁷⁶Ge crystal, a passive volume attenuating gamma-rays from the experimental hall and as an active volume, producing scintillation light at l= 128 nm. This dissertation will look to further support the Lar active veto system by investigating the material poly(ethylene 2,6 -naphthalate), (PEN).

For PEN to be used as an active material in LEGEND, it is necessary to understand its radioactivity, its optical properties and its radioluminescent properties. In this dissertation I detail the measurements I led to determine the optical attenuation length, surface reflectivity and its light response to neutrons.

PEN baseplates that were previously characterized, were installed in the LEGEND R&D effort, the post Gerda test (PGT). The PGT included 18 kg of ⁷⁶Ge crystals, with 40% PEN baseplates, and 60% silicon baseplates. A description of data collected from the PGT is included, and how the detectors performed with PEN versus silicon baseplates is described. Using simulations of the PGT, it was possible to place limits of ²³²Th contamination in the PEN baseplates as well as predict the improved veto efficiency with the active PEN versus the passive silicon.

This dissertation will evaluate PEN as a scintillating material for structural components in the 0nbb decay experiment, LEGEND by evaluating the PGT and compare simulations to experimental results. This comparison will provide insight on how PEN components can impact veto efficiency and further suppress background for future 0nbb experiments.