The Krypton String Test Cryostat: Characterizing Surface Charge-Induced Dead Layers on High Purity Germanium Detectors

Abstract

The LEGEND experiment is searching for neutrinoless double beta decay, a proposed nuclear decay that, if detected, can help explain some of the most fundamental questions in physics. LEGEND is using germanium detectors, enriched in the double beta-decaying isotope $^{76}$Ge to search for this rare process. Background reduction is crucially important to rare event searches like LEGEND and understanding these germanium detectors is a necessary guard against misunderstood backgrounds. This work describes the findings of a germanium test-stand, the Krypton String Test Cryostat (KrSTC). KrSTC uses low energy ($< 40$~keV) photons and electrons to probe the passivated surface of these detectors. Analysis of the KrSTC data shows different low energy spectra depending on detector and cryostat conditions and suggests a dead layer model investigated through simulation work. This dead layer model, accounting for surface charge collecting on the passivated surface, can be a useful tool in understanding LEGEND backgrounds in the search for neutrinoless double beta decay.