Abstract
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 5.9pm 0.6~upmu Bq/kg and 1.6pm 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.
Highlights
The SABRE experiment seeks to detect the annual modulation of the dark matter interaction rate in NaI(Tl) crystals with sufficient sensitivity to test the long standing DAMA result in a model independent way
The DAMA evidence of a clear annual modulation, which satisfies the criteria for a model-independent WIMPinduced signal [4], has reached a strong statistical significance of 12.9 σ [5], after 20 years of data taking at the Gran Sasso National Laboratory (LNGS), in Italy
While the null result of experiments based on different target materials and other direct detection techniques [6,7,8,9,10,11,12,13] cannot be reconciled with such modulation in the simplest scenario of Weakly Interacting Massive Particles (WIMPs)-nucleon elastic scattering, a NaI(Tl) annual modulation search similar to DAMA is still crucial to shed light on the origin of the positive signal [14]
Summary
The SABRE experiment seeks to detect the annual modulation of the dark matter interaction rate in NaI(Tl) crystals with sufficient sensitivity to test the long standing DAMA result in a model independent way. The SABRE concept and ambitious goal is to obtain an ultra-low background rate in the energy region of interest, namely of the order of 0.1 count/day/kg/keV, that is several times lower than the DAMA/LIBRA level. This challenging goal is achievable by developing high-purity crystals and operating them inside a liquid scintillator veto for active background rejection. In 2015, we successfully grew at RMD a 2 kg crystal with an average 39K level of 9 ± 1 ppb and with 87Rb upper limit of 0.1 ppb measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) [24,25] Based on these results, a simulation study of the background contributions expected for the SABRE PoP phase can be found in Ref. In the few months we aim at a more comprehensive characterization with the PoP setup
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