$\alpha$-event Characterization and Rejection in Point-Contact HPGe Detectors

Abstract

P-type point contact (PPC) HPGe detectors are a leading technology for rare event searches due to their excellent energy resolution, low thresholds, and multi-site event rejection capabilities. We have characterized a PPC detector's response to $\alpha$ particles incident on the sensitive passivated and p+ surfaces, a previously poorly-understood source of background. The detector studied is identical to those in the MAJORANA DEMONSTRATOR experiment, a search for neutrinoless double-beta decay ($0\nu \beta \beta$) in ${}^{76}$Ge. $\alpha$ decays on most of the passivated surface exhibit significant energy loss due to charge trapping, with waveforms exhibiting a delayed charge recovery (DCR) signature caused by the slow collection of a fraction of the trapped charge. The DCR is found to be complementary to existing methods of $\alpha$ identification, reliably identifying $\alpha$ background events on the passivated surface of the detector. We demonstrate effective rejection of all surface $\alpha$ events (to within statistical uncertainty) with a loss of only 0.2% of bulk events by combining the DCR discriminator with previously-used methods. The DCR discriminator has been used to reduce the background rate in the $0\nu \beta \beta$ region of interest window by an order of magnitude in the MAJORANA DEMONSTRATOR, and will be used in the upcoming LEGEND-200 experiment.

Accepted for publication in European Journal of Physics C

References in corpus (5)

• Final Results of GERDA on the Search for Neutrinoless Double-$\beta$ Decay
• LEGEND-1000 Preconceptual Design Report
• Multi-site event discrimination for the MAJORANA DEMONSTRATOR
• Impact of ADC non-linearities on the sensitivity to sterile keV neutrinos with a KATRIN-like experiment
• MAJORANA Collaboration's Experience with Germanium Detectors

Cited by in corpus (11)

• Final Result of the MAJORANA DEMONSTRATOR's Search for Neutrinoless Double-$\beta$ Decay in ${}^{76}$Ge
• The MAJORANA DEMONSTRATOR Readout Electronics System
• Energy Calibration of Germanium Detectors for the MAJORANA DEMONSTRATOR
• Development of low-threshold detectors for low-mass dark matter searches with a p-type germanium detector operated at cryogenic temperature
• Interpretable Boosted Decision Tree Analysis for the Majorana Demonstrator
• Final Results of the MAJORANA DEMONSTRATOR's Search for Double-Beta Decay of ${}^{76}$Ge to Excited States of ${}^{76}$Se
• A novel wide-angle Compton Scanner setup to study bulk events in germanium detectors
• Constraints on inelastic dark matter from the CDEX-1B experiment
• Rare multi-nucleon decays with the full data sets of the Majorana Demonstrator
• Identification and simulation of surface alpha events on passivated surfaces of germanium detectors and the influence of metalisation
• Machine learning-powered data cleaning for LEGEND: a semi-supervised approach using affinity propagation and support vector machines