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 \(^{39}\)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 \(^{226}\)Ra and \(^{228}\)Th inside the crystal to be \(5.9\pm 0.6~\upmu \)Bq/kg and \(1.6\pm 0.3~\upmu \)Bq/kg, respectively, which would indicate a contamination from \(^{238}\)U and \(^{232}\)Th at part-per-trillion level. We measured an activity of 0.51 ± 0.02 mBq/kg due to \(^{210}\)Pb out of equilibrium and a \(\alpha \) 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 \(\sim \)1 count/day/kg/keV in the [5–20] keV region.

超纯NaI（Tl）晶体是长期独立的DAMA结果的独立于模型的验证的关键元素，也是寻找暗物质相互作用的年度调制特征的有力手段。SABER合作多年来一直在开发用于减少内在背景的尖端技术。在本文中，我们报告了在LNGS的地下无源屏蔽装置中进行的3.4千克晶体NaI-33的首次表征。通过质谱测定，NaI-33的记录的低K （^ {39} \） K污染为4.3±0.2 ppb。我们在59.5 keV下测得的光产率为11.1±0.2光电子/ keV，能量分辨率为13.2％（FWHM / E）。我们评估了\（^ {226} \） Ra和\（^ {228} \）的活动晶体内部分别为\（5.9 \ pm 0.6〜\ upmu \） Bq / kg和\（1.6 \ pm 0.3〜\ upmu \） Bq / kg，这表明来自\（^ {238} \） U和\（^ {232} \） Th处于万亿分之一的水平。由于在5304 keV下\（^ {210} \） Pb不平衡，并且\（\ alpha \）猝灭因子为0.63±0.01，因此我们测得的活性为0.51±0.02 mBq / kg 。我们举例说明了为抑制能谱下部的电子噪声而开发的分析技术。基于切割的策略和多变量方法表明，在[5-20] keV区域中，由于晶体固有的放射性，\（\ sim \） 1个计数/天/ kg / keV的比率。