Development of low-background Cs2ZrCl6 detectors to study rare decays in Zr isotopes

In this work, we present the first complex study of CsZrCl (CZC) scintillating crystals in terms of their chemical purity, crystal radiopurity, scintillating performance and pulse-shape discrimination ability. An ICP-MS measurement exhibits a high purity of the CZC material (overall impurity level less than 10 g/g). The CZC crystals show a very low content of radionuclides from U/Th natural decay chains. For K and Cs only limits at the level few mBq/kg were set after more than 650 h of measurement with a high purity germanium (HPGe) detector. Nevertheless, they contained cosmogenic Cs with an activity of about 50 mBq/kg. The measurements with two CZC crystals (11 and 24 g) as scintillating detectors over 456.5 d in the low-background DAMA/CRYS facility, deep underground at LNGS, supported a substantial radiopurity of these crystals leading to a counting rate of 0.17 (kgkeVyr) at the of Zr (3.35 MeV). These detectors show a high particle-discrimination ability for and particles. Limits on the half-lives of the double beta decays (DBD) of Zr and of Zr with and without neutrinos emission were also set at the level – yr (90 C.L.). The detailed analyses of the internal background components will also be useful for further developments of CsZrCl low-background scintillating detectors and for the optimization of the experimental conditions.