Virtual segmentation of a small contact HPGe detector: inference of hit positions of single-site events via pulse shape analysis

TL;DR

This study demonstrates a method to virtually segment a small contact HPGe detector into layers using pulse shape analysis, enabling better background suppression for rare event searches like neutrinoless double beta decay.

Contribution

The paper introduces a novel pulse shape analysis algorithm to virtually segment a germanium detector into layers, improving background discrimination in rare event experiments.

Findings

Inner layer volume is approximately 47.2% of total volume.

Event rate in the 0νββ signal region is reduced by 12% using virtual segmentation.

Good agreement between experimental data and simulation for SSE spectra.

Abstract

Exploring hit positions of recorded events can help to understand and suppress backgrounds in rare event searching experiments. In this study, we virtually segment a small contact P-type high purity germanium detector (HPGe) into two layers. Single-site events (SSEs) in each layer are selected by an algorithm based on two pulse shape parameters: the charge pulse drift time ($T_{Q}$) and current pulse rise time ($T_{I}$). To determine the shapes and volumes of the two layers, a Th-228 source is placed at top and side positions to irradiate the detector. The double escape peak events from 2614.5 keV $\gamma$-ray are selected as typical SSEs, their numbers in the two layers are used to calculate the volumes and shapes of those layers. Considering the statistical and systematic uncertainties, the inner layer volume is evaluated to be 47.2\%$\pm$0.26(stat.)\%$\pm$0.22(sys.)\% of the total sensitive volume. We extend our analysis for SSEs in 1400-2100 keV, the spectra of inner layer events acquired from experimental data using the selection algorithm are in good agreement with those from the simulation. For sources outside the HPGe detector, the outer layer can act as a shielding for the inner layer. Selecting the inner layer as the analysis volume can reduce the externalbackground in the signal region of Ge-76 neutrinoless double beta (0$\nu\beta\beta$) decay. We use the Th-228 source to evaluate the background suppression power of the virtual segmentation. After performing the single and multi-site event discrimination, the event rate in the 0$\nu\beta\beta$ signal region can be further suppressed by 12\% by selecting the inner layer as the analysis volume. The virtual segmentation could be used to efficiently suppress surface background like electrons from Ar-42/K-42 decay in 0$\nu\beta\beta$ experiments using germanium detector immersed in liquid argon.