Light Collection of Some Molybdate Crystal Absorbers for Cryogenic Calorimeters at Millikelvin Temperatures

Scintillating molybdate crystals recently got significant attention for their applications in rare event searches, such as neutrinoless double beta decay experiments. In this article, we report on the light collection of cryogenic calorimeters operating at 10 mK, using CaMoO ₄ (CMO), Li $_{2}^{100}$ MoO ₄ (LMO), and Na ₂ Mo ₂ O ₇ (NMO) crystal absorbers. Scintillation signals from gamma rays and alpha particles are studied. Germanium (Ge) and silicon (Si) wafers, which work as light absorbers, coupled to metallic magnetic calorimeters (MMC), are used as scintillation light detectors. By comparing the scintillation signals with “direct hit” X-ray signals of known energies, which occurred in the light absorber, we assess the light collection of the crystal scintillation. Eight keV copper X-rays are analyzed in the case of both the Ge and Si wafer detectors. The light measurement with LMO and NMO crystals at millikelvin temperatures is $9.6\times $ and $7.6\times $ less compared with the CMO for 2.615-MeV gamma rays from ²⁰⁸ Tl. Additionally, we found that for NMO, the detected scintillation light for alpha particles is quenched by a factor of 0.06 compared to that of gamma rays for a given energy by investigating the light signal from 2.615-MeV gamma rays and 5.4-MeV alpha particles. For the CMO, in the same way, the quenching factor is measured to be 0.22. The quenching factor for 4.785-MeV alpha particles ( ⁶ Li neutron capture) in LMO crystal is about 0.28 compared to the scintillation from 2.615-MeV gamma rays.