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We present here a characterization of the low background NaI(Tl) crystal NaI-33 based on a period of almost one year of data taking (891 kg$$\times $$$\times$days exposure) in a detector configuration with no use of organic scintillator veto. This remarkably radio-pure crystal already showed a low background in the SABRE Proof-of-Principle (PoP) detector, in the low energy region of interest (1–6 keV) for the search of dark matter interaction via the annual modulation signature. As the vetoable background components, such as$$^{40}$$${}^{40}$K, are here sub-dominant, we reassembled the PoP setup with a fully passive shielding. We upgraded the selection of events based on a Boosted Decision Tree algorithm that rejects most of the PMT-induced noise while retaining scintillation signals with > 90% efficiency in 1–6 keV. We find an average background of 1.39 ± 0.02 counts/day/kg/keV in the region of interest and a spectrum consistent with data previously acquired in the PoP setup, where the external veto background suppression was in place. Our background model indicates that the dominant background component is due to decays of$$^{210}$$${}^{210}$Pb, only partly residing in the crystal itself. The other location of$$^{210}$$${}^{210}$Pb is the reflector foil that wraps the crystal. We now proceed to design the experimental setup for the physics phase of the SABRE North detector, based on an array of similar crystals, using a low radioactivity PTFE reflector and further improving the passive shielding strategy, in compliance with the new safety and environmental requirements of Laboratori Nazionali del Gran Sasso.

 

The dark matter interpretation of the DAMA/LIBRA annual modulation signal represents a long-standing open question in astroparticle physics. The SABRE experiment aims to test such claim, bringing the same detection technique to an unprecedented sensitivity. Based on ultra-low background NaI(Tl) scintillating crystals like DAMA, SABRE features a liquid scintillator Veto system, surrounding the main target, and it will deploy twin detectors: one in the Northern hemisphere at Laboratori Nazionali del Gran Sasso (LNGS), Italy and the other in the Stawell Underground Physics Laboratory (SUPL), Australia, first laboratory of this kind in the Southern hemisphere. The first very-high-purity crystal produced by the collaboration was shipped to LNGS in 2019 for characterization. It features a potassium contamination, measured by mass spectroscopy, of the order of 4 ppb, about three times lower than DAMA/LIBRA crystals. The first phase of the SABRE experiment is a Proof-of-Principle (PoP) detector featuring one crystal and a liquid scintillator Veto, at LNGS. This contribution will present the results of the stand-alone characterization of the first SABRE high-purity crystal, as well as the status of the PoP detector, commissioned early in the summer of 2020. 

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$$5.9\pm 0.6~\upmu $$$5.9\pm 0.6\mu$Bq/kg and$$1.6\pm 0.3~\upmu $$$1.6\pm 0.3\mu$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$$\alpha $$$\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$$\sim $$$\sim$1 count/day/kg/keV in the [5–20] keV region.

 

Abstract SABRE is a dark matter direct detection experiment aiming to measure the annual modulation of the dark matter interaction rate in NaI(Tl) crystals. SABRE focuses on the achievement of an ultra-low background rate operating high-purity NaI(Tl) crystals in a liquid scintillator veto for active background rejection. Moreover, twin experiments will be located in both Northern and Southern hemispheres (Italy and Australia) to disentangle any possible contribution from seasonal or site-related effects. In this article the results of the first measurements with a NaI(Tl) crystal for the SABRE experiment performed at LNGS are presented. 

Abstract SABRE is a dark matter direct detection experiment based on NaI(Tl) scintillating crystals. The primary goal of the experiment is to test the dark matter interpretation of the DAMA/LIBRA annual modulation signal. To reach its purpose, SABRE will operate an array of ultra-low background NaI(Tl) crystals within an active veto, based on liquid scintillator. Finally two twin detectors will be used, one in the northern hemisphere at Laboratori Nazionali del Gran Sasso, Italy (LNGS) and the other, first of its kind, in the southern hemisphere, in the Stawell Underground Physic Laboratory (SUPL). The collaboration has successfully developed a NaI(Tl) crystal with the impressive potassium content of about 4 ppb, according to the mass spectroscopy measurements. A value that, if confirmed, would be about 3 times lower than the DAMA/LIBRA crystals one. The first phase of the SABRE experiment, called SABRE Proof of Principle (PoP), aims to prove the achieved radiopurity by direct measurement of crystals at LNGS. This work reports the status of the PoP setup and the recent progresses on the development of low radioactivity NaI(Tl) crystals.