Test of 116CdWO4 and Li2MoO4 scintillating bolometers in the CROSS underground facility with upgraded detector suspension

In preparation for the CROSS experiment to search for double-beta (2β) decay of 100Mo, we upgraded a low-background facility at the Canfranc underground laboratory (Spain) by installing a magnetic damping-based detector suspension inside a pulse-tube cryostat. We tested the suspension in low-temperature (15 mK) measurements with two scintillating bolometers made of large-volume crystals of 116CdWO4 (reference of the set-up) and Li2MoO4 (R&D of the CLYMENE project in view of the CUPID 2β experiment), each coupled to a thin Ge bolometric light detector. Despite the evidence of a residual pulse-tube-induced noise, picked up by cabling, we achieved high performance with all tested devices. In particular, the energy resolution for 2615 keV γ-ray measured with both 116CdWO4 and Li2MoO4 bolometers is ∼6 keV FWHM, among the best-reported results for thermal detectors based on such compounds representing a great interest to searches for 2β decay in 116Cd (Q-value is 2813 keV) and 100Mo (3034 keV) respectively. Highly efficient particle identification (α vs. β/γ) is achieved also with the scintillating bolometer made of a low light yield crystal (Li2MoO4), thanks to the low noise (10 eV RMS) exhibited by the light detector exploiting the Neganov-Trofimov-Luke effect for signal amplification. We also found a rather high level of radiopurity in the Li2MoO4 crystal; only traces of 210Po and 226Ra were detected (∼0.1 mBq/kg each), while the 228Th activity is expected to be at least an order of magnitude lower, as well as a 40K activity is below 6 mBq/kg.