Characterization of an ultra-high purity NaI(Tl) crystal scintillator with the SABRE Proof-of-Principle detector

31 Aug 2021, 17:10
Talk in parallel session Dark Matter and its detection Discussion Panel Dark Matter 3


Ambra Mariani (Princeton University)


The SABRE (Sodium-iodide with Active Background REjection) experiment aims to detect the annual modulation of the dark matter interaction rate by means of ultra-high purity NaI(Tl) crystals immersed in a liquid scintillator (LS) active veto. It focuses on the achievement of a very low background to carry out a model-independent test of the long-standing DAMA/LIBRA result with sufficient sensitivity using the same target material. The two hemispheres approach, one detector placed at the Gran Sasso National Laboratory (LNGS), in Italy, and the other at the Stawell Underground Physics Laboratory (SUPL), in Australia, allows to disentangle seasonal or site-related effects from the dark matter annually modulated signal. The SABRE experiment has recently completed a Proof-of-Principle (PoP) phase, which was devoted to assess the radiopurity of the crystals as well as the efficiency of the LS veto. In this talk, the new results on the radiopurity of a 3.4-kg NaI(Tl) crystal scintillator grown within the SABRE Collaboration, and operated underground in the SABRE-PoP setup, will be presented and discussed. The amount of potassium content in the crystal, determined by direct counting of $^{40}$K, is found to be < 4.7 ppb at 90% C.L., lowest level ever achieved for NaI(Tl) crystals. With active veto, the average background rate in the [1-6] keV energy region-of-interest is 1.20 ± 0.05 counts/day/kg/keV, which is, for the first time, comparable with DAMA/LIBRA-phase1. Our background model indicates that the rate is dominated by $^{210}$Pb and that about half of this contamination is located in the PTFE reflector wrapped around the crystal. Ongoing developments aimed at a further reduction of radioactive contaminants in the crystal will be also discussed. A projected background rate lower than ~0.2 counts/day/kg/keV in the region-of-interest is within reach, and represents a benchmark for the development of next-generation NaI(Tl) detector arrays for the direct detection of dark matter particles.

Reference to paper (DOI or arXiv) arXiv:2105.09225

Primary author

Ambra Mariani (Princeton University)

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