Characterization of SABRE crystal NaI-33 with direct underground counting

Ultra-pure NaI(Tl) crystals are the key element for a model-independent verification of the long standing DAMA result and a powerful means to search for the annual modulation signature of dark matter interactions. The SABRE collaboration has been developing cutting-edge techniques for the reduction of intrinsic backgrounds over several years. In this paper we report the first characterization of a 3.4 kg crystal, named NaI-33, performed in an underground passive shielding setup at LNGS. NaI-33 has a record low ³⁹ 39 K contamination of 4.3 ± 0.2 ppb as determined by mass spectrometry. We measured a light yield of 11.1 ± 0.2 photoelectrons/keV and an energy resolution of 13.2% (FWHM/E) at 59.5 keV. We evaluated the activities of ²²⁶ 226 Ra and ²²⁸ 228 Th inside the crystal to be 5.9± 0.6~ 5.9 ± 0.6 μ Bq/kg and 1.6± 0.3~ 1.6 ± 0.3 μ Bq/kg, respectively, which would indicate a contamination from ²³⁸ 238 U and ²³² 232 Th at part-per-trillion level. We measured an activity of 0.51 ± 0.02 mBq/kg due to ²¹⁰ 210 Pb out of equilibrium and a α α quenching factor of 0.63 ± 0.01 at 5304 keV. We illustrate the analyses techniques developed to reject electronic noise in the lower part of the energy spectrum. A cut-based strategy and a multivariate approach indicated a rate, attributed to the intrinsic radioactivity of the crystal, of ~ ∼ 1 count/day/kg/keV in the [5–20] keV region.