Assessment of Cs₂HfCl₆ crystal applicability as low-temperature scintillating bolometers by their thermodynamic characteristics

International audienceThe first study of some thermodynamic properties of a Cs2HfCl6 (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