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 MajoranaDemonstrator experiment, a search for neutrinoless double-beta decay (0nu 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 0nu beta beta region of interest window by an order of magnitude in the MajoranaDemonstrator and will be used in the upcoming LEGEND-200 experiment.
Highlights
We demonstrate effective rejection of all surface α events with a loss of only 0.2% of bulk events by combining the delayed charge recovery (DCR) discriminator with previously-used methods
P-type point contact (PPC) High-Purity Germanium (HPGe) detectors [6] are a key technology for rare event searches, capable of detection thresholds below 1 keV and full-width at half-maximum (FWHM) energy resolutions of 0.12% (2.5 keV) at the 0νββ region-of-interest of 2039 keV [7]
This paper presented data taken with a p-type point contact detector produced by ORTEC that reproduces the design of the Majorana Demonstrator enriched detectors
Summary
The discovery of neutrinoless double-beta decay (0νββ) would indicate that neutrinos are Majorana particles and that lepton number is not conserved. It could provide information on the absolute mass scale of the neutrino, the source of the neutrino’s mass, and the origin of the matter/antimatter asymmetry of the universe [1–3] This radioactive decay would occur only rarely; sensitivity limits from current experiments indicate that the half-life is over 1026 years [4,5]. P-type point contact (PPC) High-Purity Germanium (HPGe) detectors [6] are a key technology for rare event searches, capable of detection thresholds below 1 keV and full-width at half-maximum (FWHM) energy resolutions of 0.12% (2.5 keV) at the 0νββ region-of-interest of 2039 keV [7] They have been used to search for low-energy nuclear recoils from external sources in dark matter searches, [8] and have been proposed for use in coherent neutrino-nuclear scattering experiments [9]. We performed a dedicated study of α interactions on this surface, leading to more reliable models of the α energy spectrum and the distinctive pulse-shape characteristics of these signals
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