Speaker
Description
The SABRE project aims to produce ultra-low background NaI(Tl) scintillating detectors to carry out a model-independent search for dark matter through the annual modulation signature, with an unprecedented sensitivity to confirm or refute the DAMA/LIBRA claim. The ultimate goal of SABRE is to operate two independent NaI(Tl) crystal arrays located in the northern (SABRE North) and southern (SABRE South) hemispheres to identify possible contributions to the modulation from seasonal or site-related effects.
As a large fraction of the background in the 1-6 keV energy region-of-interest (ROI) for dark matter search come from radioactive contaminants in the crystal themselves, SABRE North has carried out an extensive R&D on the production of ultra radio-pure NaI(Tl) crystals. Direct counting of beta and gamma particles with the SABRE Proof-of-Principle detector, equipped with a liquid scintillator active veto and operated at the Gran Sasso National Laboratory (LNGS) has already demonstrated an average background rate of 1.20 $\pm$ 0.05 counts/day/kg/keV for the so-called NaI-33 crystal, which is a breakthrough since the DAMA/LIBRA experiment. In particular, the amount of potassium contamination is found to be lower than 4.7 ppb at 90% CL, lowest ever achieved for NaI(Tl) crystals.
Data acquired for about one year with the NaI-33 detector into a purely passive shielding have shown that, if the crystal vetoable internal contaminations are as low as in the NaI-33, the active veto is no longer a crucial feature to achieve the required sensitivity. Indeed, our background model indicates that the rate is dominated by $^{210}$Pb decays and that a large fraction of this contamination is located in the PTFE reflector wrapping the crystal. Beside the replacement of this material, ongoing developments of the crystal manufacture are aimed at a further reduction of the intrinsic background. The present results represent a benchmark for the development of next-generation NaI(Tl) detectors for the direct detection of dark matter particles, with a projected background rate lower than $\sim$0.3 counts/day/kg/keV. With this level of background an array of NaI(Tl) scintillating crystals with a total mass of just a fraction of the present generation experiments can achieve the ultimate verification of the DAMA result in about three years.
