Identification and simulation of surface alpha events on passivated surfaces of germanium detectors and the influence of metalisation

TL;DR

This paper investigates alpha surface events in germanium detectors, analyzing charge trapping effects, verifying pulse-shape analysis techniques, and modeling charge trapping, while also exploring the impact of metallization on low-energy gamma interactions.

Contribution

It introduces a detailed analysis of charge trapping on passivated surfaces, verifies pulse-shape identification methods, and presents a new model for charge trapping effects in germanium detectors.

Findings

Charge trapping depends on the crystal axes.

Pulse-shape analysis can identify alpha events effectively.

Metallisation influences low-energy gamma interactions near the surface.

Abstract

Events from alpha interactions on the surfaces of germanium detectors are a major contribution to the background in germanium-based searches for neutrinoless double-beta decay. Surface events are subject to charge trapping, affecting their pulse shape and reconstructed energy. A study of alpha events on the passivated end-plate of a segmented true-coaxial n-type high-purity germanium detector is presented. Charge trapping is analysed in detail and an existing pulse-shape analysis technique to identify alpha events is verified with mirror pulses observed in the non-collecting channels of the segmented test detector. The observed radial dependence of charge trapping confirms previous results. A dependence of the probability of charge trapping on the crystal axes is observed for the first time. A first model to describe charge trapping effects within the framework of the simulation software SolidStateDetectors$.$jl is introduced. The influence of metalisation on events from low-energy gamma interactions close to the passivated surface is also presented.