Surface Characterization of P-Type Point Contact Germanium Detectors

Frank Edzards,Michael Willers,Xiang Liu,Iris Abt,David C Radford,Chris Gooch,Susanne Mertens,Björn Lehnert,Oliver Schulz,Lukas Hauertmann

doi:10.3390/particles4040036

Abstract

P-type point contact (PPC) germanium detectors are used in rare event and low-background searches, including neutrinoless double beta (0νββ) 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.

Highlights

The response of a p-type point contact (PPC) germanium detector to alpha and beta particles was studied in detail to better understand background events as occurring in experiments searching for neutrinoless double beta decay
The results of the measurements in the vacuum test facility G ALATEA demonstrate that the structure of events on the passivated detector surface can be explained by effects like surface charges
For both alpha and beta surface events, a radius-dependent reduction of the energy was observed which can be explained by assuming the presence of a negative surface charge

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The observation of neutrinoless double beta (0νββ) decay could have major implications on our understanding of the origin of matter in our universe. The decay violates lepton number conservation by two units, and the search for it is the most practical way to ascertain whether neutrinos are Majorana particles, i.e., their own antiparticles (ν = ν). Together with cosmological observations and direct neutrino mass measurements, it could provide information on the absolute neutrino mass scale and ordering, for recent reviews see [1,2]

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