Developments, features and perspectives of crystal scintillators of the Cs₂MCl₆family (M = Hf or Zr) to search for rare processes

Recently, there has been considerable interest in the development of crystal scintillators of the Cs₂MCl₆ family of metal hexachlorides (M = Hf or Zr) due to their exceptional properties: high light yield (up to 40000 photons/ MeV ), good linearity in the energy response, excellent energy resolution (< 3.5% at 662 keV in the best configuration) and excellent ability to discriminate between pulse shapes (PSD) of β(γ) and α particles. One Cs₂HfCl₆ (CHC) crystal scintillator was measured, over 2848 h of data taking, deep underground in the STELLA laboratory at the Gran Sasso National Laboratory of the INFN, Italy. Its residual radioactive contaminants were studied and are presented here. The total α activity of the detector was at the level of 7.8(3) mBq/kg. A measurement using two Cs₂ZrCl₆ (CZC) crystal scintillators (11 g and 24 g) was performed in the DAMA/CRYS low-background setup deep underground at LNGS. Chemical purity, residual radioactive contaminants, scintillation, and PSD performance are discussed in this paper. The low-background measurements over 456.5 days demonstrated the crystals’ high radiopurity, showing a counting rate of 0.17(kg·keV·y)^-1 at the Q_2β = 3.35 MeV of ⁹⁶Zr. Another measurement was subsequently carried out using three new CZC crystals and one CHC crystal in optimized geometry. The crystal growth technique, raw material purification, and post-growth material treatment are discussed here. Moreover, the three CZC crystals were grown using starting materials with different purities to study their resulting characteristics and were encapsulated using a silicone-based sealant. The results obtained are presented in this paper.