Phonon-mediated crystal detectors with metallic film coating capable of rejecting and events induced by surface radioactivity
I.C. Bandac🇪🇸 · A.S. Barabash🇷🇺 · L. Bergé🇫🇷 · Ch. Bourgeois🇫🇷 · J.M. Calvo-Mozota🇪🇸 · P. Carniti🇮🇹 · M. Chapellier🇫🇷 · M. de Combarieu🇫🇷 · I. Dafinei🇮🇹 · F.A. Danevich🇺🇦 · L. Dumoulin🇫🇷 · F. Ferri🇫🇷 and 24 other authors
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 and 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 mm LiMoO 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 mm face. Discrimination capabilities were tested with U sources emitting both and 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.