Assessment of CsHfCl crystal applicability as low-temperature scintillating bolometers by their thermodynamic characteristics

The first study of some thermodynamic properties of a Cs 2 HfCl 6 (CHC) scintillating crystal has been performed in a wide temperature range of 5-300 K. The CHC crystal exhibits a high thermal expansion coefficient (5.17 × 10 −5 at 171 K), along with an extremely low heat conductivity (1.710 W m −1 K −1 at 50 K). A high heat capacity (1.22 J kg −1 K −1 at 3.8 K) along with a drastically low free mean path for phonons (about 26 Å) at temperatures around 50 K makes the CHC crystal unsuitable as a target material for cryogenic detectors. The results of the first operation of the CHC crystal as a cryogenic detector at sub-Kelvin temperatures clearly show the unfavorable phonon propagation properties of the material. On the other hand, the CHC crystal demonstrates excellent scintillating characteristics with a relatively high light yield under gamma irradiation (the detected signal is about 5200 photons per MeV). The refractive index within the (1.66-1.67) interval for light in visible and near-infrared spectral ranges was measured at room temperature. A cost- and time-effective approach for new crystalline material characterization as a target for cryogenic detectors based on the evaluation of the thermodynamic properties of the material is proposed. Measurements of thermodynamics characteristics such as heat conductivity and specific heat, along with calculations of the phonon mean free path allowing to rapidly characterize the performance of new scintillating material as a scintillating bolometer in time-cost-man power effective manner in comparison to cryogenic measurements in a dilution refrigerator.