Recent progress in Cs₂HfCl₆ (CHC) crystal production achieved within the last five years is presented. Various aspects have been analyzed, including the chemical purity of raw materials, purification methods, optimization of the growth and thermal conditions, crystal characterization, defect structure, and internal radioactive background. Large volume, crack-free, and high quality CHC crystals with an ultimate scintillating performance were produced as a result of such extensive research and development (R & D) program. For example, the CHC crystal sample with dimensions ∅23 × 30 mm³ demonstrates energy resolution of 3.2% FWHM at 662 keV, the relative light output at the level of 30,000 ph/MeV and excellent linearity down to 20 keV. Additionally, this material exhibits excellent pulse shape discrimination ability and low internal background of less than 1 Bq/kg. Furthermore, attempts to produce a high quality CHC crystal resulted in research on this material optimization by constitution of either alkali ions (Cs to Tl), or main element (Hf to Zr), or halogen ions (Cl to Br, I, or their mixture in different ratio), as well as doping with various active ions (Te⁴⁺, Ce³⁺, Eu³⁺, etc.). This leads to a range of new established scintillating materials, such as Tl₂HfCl₆, Tl₂ZrCl₆, Cs₂HfCl₄Br₂, Cs₂HfCl₃Br₃, Cs₂ZrCl₆, and Cs₂HfI₆. To exploit the whole potential of these compounds, detailed studies of the material’s fundamental properties, and understanding of the variety of the luminescence mechanisms are required. This will help to understand the origin of the high light yield and possible paths to further extend it. Perspectives of CHC crystals and related materials as detectors for rare nuclear processes are also discussed.

Crystals of metal hexachlorides Cs₂MCl₆ (M = Hf or Zr) have recently emerged as promising materials for scintillation applications due to their excellent energy resolution.

We explore the reach of low-background experiments made of small quantities of heavy nuclear isotopes in probing the parameter space of inelastic dark matter that is kinematically inaccessible to classic direct detection experiments. Through inelastic scattering with target nuclei, dark matter can yield a signal via either nuclear recoil or nuclear excitation. We present new results based on this approach, using data from low-energy gamma quanta searches in low-background experiments with Hf and Os metal samples and measurements with ${\mathrm{CaWO}}_4$ and ${\mathrm{PbWO}}_4$ crystals as scintillating bolometers. We place novel bounds on WIMPy inelastic dark matter up to mass splittings of about 640 keV and provide forecasts for the reach of future experiments.

A new search for the decay modes of the fourfold forbidden nonunique decay of ${}^{50}\mathrm{V}$ has been performed at the Gran Sasso Underground Laboratory (LNGS). In total, an exposure of 197 kg d has been accumulated. The half-life for the electron capture into the first excited state of ${}^{50}\mathrm{Ti}$ has been measured with the highest precision to date as $2.{67}_{-0.18}^{+0.16}\times {10}^{17}$ yr (68% C.I.) in which systematics uncertainties dominate. The search for the $\beta$ decay into the first excited state of ${}^{50}\mathrm{Cr}$ resulted in a lower limit of $1.9\times {10}^{19}$ yr (90% C.I.), which is an improvement of almost one order of magnitude compared to existing results. The sensitivity of the new measurement is now in the region of theoretical predictions.