Phonon-mediated crystal detectors with metallic film coating capable of rejecting $\alpha$ and $\beta$ events induced by surface radioactivity

TL;DR

This paper introduces a novel method using metallic film coatings on crystal detectors to effectively distinguish and reject surface-induced alpha and beta background events, enhancing the sensitivity of rare-event physics experiments.

Contribution

The study demonstrates that coating crystal detectors with metallic films modifies phonon signals, enabling efficient discrimination of surface events from internal events in rare-event searches.

Findings

Surface event discrimination achieved with coated crystals.

Effective identification of surface alpha and beta events at millimeter depths.

Potential for significant background reduction in neutrinoless double-beta decay experiments.

Abstract

Phonon-mediated particle detectors based on single crystals and operated at millikelvin temperatures are used in rare-event experiments for neutrino physics and dark-matter searches. In general, these devices are not sensitive to the particle impact point, especially if the detection is mediated by thermal phonons. In this Letter, we demonstrate that excellent discrimination between interior and surface $\beta$ and $\alpha$ events can be achieved by coating a crystal face with a thin metallic film, either continuous or in the form of a grid. The coating affects the phonon energy down-conversion cascade that follows the particle interaction, leading to a modified signal shape for close-to-film events. An efficient identification of surface events was demonstrated with detectors based on a rectangular $20 \times 20 \times 10$ mm$^3$ Li$_2$MoO$_4$ crystal coated with a Pd normal-metal film (10~nm thick) and with Al-Pd superconductive bi-layers (100~nm-10~nm thick) on a $20 \times 20$ mm$^2$ face. Discrimination capabilities were tested with $^{238}$U sources emitting both $\alpha$ and $\beta$ particles. Surface events are identified for energy depositions down to millimeter-scale depths from the coated surface. With this technology, a substantial improvement of the background figure can be achieved in experiments searching for neutrinoless double-beta decay.