Delayed charge recovery discrimination of passivated surface alpha events in P-type point-contact detectors

TL;DR

This paper introduces a new method to identify and reject passivated surface alpha events in germanium detectors, improving background discrimination for neutrinoless double-beta decay searches.

Contribution

The study develops a filter based on delayed charge recovery characteristics to distinguish surface alpha events from true signals in P-type point-contact detectors.

Findings

The filter effectively identifies passivated surface alpha events.

It reduces background contamination in the detector data.

The method enhances the sensitivity of neutrinoless double-beta decay searches.

Abstract

The Majorana Demonstrator searches for neutrinoless double-beta decay of $^{76}$Ge using arrays of high-purity germanium detectors. If observed, this process would demonstrate that lepton number is not a conserved quantity in nature, with implications for grand-unification and for explaining the predominance of matter over antimatter in the universe. A problematic background in such large granular detector arrays is posed by alpha particles. In the Majorana Demonstrator, events have been observed that are consistent with energy- degraded alphas originating on the passivated surface, leading to a potential background contribution in the region-of-interest for neutrinoless double-beta decay. However, it is also observed that when energy deposition occurs very close to the passivated surface, charges drift through the bulk onto that surface, and then drift along it with greatly reduced mobility. This leads to both a reduced prompt signal and a measurable change in slope of the tail of a recorded pulse. In this contribution we discuss the characteristics of these events and the development of a filter that can identify the occurrence of this delayed charge recovery, allowing for the efficient rejection of passivated surface alpha events in analysis.