A Novel Mechanical Design of a Bolometric Array for the CROSS Double-Beta Decay Experiment

The CROSS experiment will search for neutrinoless double-beta decay using aspecific mechanical structure to hold thermal detectors. The design of thestructure was tuned to minimize the background contribution, keeping an optimaldetector performance. A single module of the structure holds two scintillatingbolometers (with a crystal size of 45x45x45 mm and a Ge slab facing thecrystal's upper side) in the Cu frame, allowing for a modular construction of alarge-scale array. Two designs are released: the initial Thick version containsaround 15ratio is reduced to  6extensively at aboveground (IJCLab, France) and underground (LSC, Spain)laboratories. In particular, at LSC we used a pulse-tube-based CROSS facilityto operate a 6-crystal array of LMOs enriched/depleted in ^100Mo. Thetested LMOs show high spectrometric performance in both designs; notably, themeasured energy resolution is 5–7 keV FWHM at 2615 keV γs, nearby theQ-value of ^100Mo (3034 keV). Due to the absence of a reflective cavityaround LMOs, a low scintillation signal is detected by Ge bolometers:  0.3 keV( 150 photons) for 1-MeV γ(β) LMO-event. Despite that, anacceptable separation between α and γ(β) events is achievedwith most devices. The highest efficiency is reached with light detectors inthe Thick design thanks to a lower baseline noise width (0.05–0.09 keV RMS)when compared to that obtained in the Slim version (0.10–0.35 keV RMS). Giventhe pivotal role of bolometric photodetectors for particle identification andrandom coincidences rejection, we will use the structure here described withupgraded light detectors, featuring thermal signal amplification via theNeganov-Trofimov-Luke effect, as also demonstrated in the present work.