Surface characterization of p-type point contact germanium detectors

TL;DR

This study investigates how p-type point contact germanium detectors respond to surface events, revealing charge accumulation effects and pulse shape features that aid in discriminating surface background signals in rare event searches.

Contribution

The paper provides detailed analysis of surface effects in PPC germanium detectors, highlighting charge build-up and pulse shape discrimination techniques for improved background rejection.

Findings

Surface charge accumulation causes radial energy degradation.

Pulse shape features enable discrimination of surface alpha events.

Understanding surface response improves detector background rejection.

Abstract

P-type point contact (PPC) germanium detectors are used in rare event and low-background searches, including neutrinoless double beta (0vbb) decay, low-energy nuclear recoils, and coherent elastic neutrino-nucleus scattering. The detectors feature an excellent energy resolution, low detection thresholds down to the sub-keV range, and enhanced background rejection capabilities. However, due to their large passivated surface, separating the signal readout contact from the bias voltage electrode, PPC detectors are susceptible to surface effects such as charge build-up. A profound understanding of their response to surface events is essential. In this work, the response of a PPC detector to alpha and beta particles hitting the passivated surface was investigated in a multi-purpose scanning test stand. It is shown that the passivated surface can accumulate charges resulting in a radial-dependent degradation of the observed event energy. In addition, it is demonstrated that the pulse shapes of surface alpha events show characteristic features which can be used to discriminate against these events.