High sensitivity characterization of an ultra-high purity NaI(Tl) crystal scintillator with the SABRE proof-of-principle detector

TL;DR

This study demonstrates ultra-high purity NaI(Tl) crystals with record low potassium contamination and background rates, advancing the development of sensitive dark matter detectors.

Contribution

It reports the lowest potassium contamination in NaI(Tl) crystals and achieves a background rate comparable to leading dark matter experiments, with ongoing improvements planned.

Findings

Potassium contamination as low as 2.2±1.5 ppb.

Background rate of 1.20±0.05 counts/day/kg/keV in 1-6 keV.

Projected background rate below 0.2 counts/day/kg/keV.

Abstract

We present new results on the radiopurity of a 3.4-kg NaI(Tl) crystal scintillator operated in the SABRE proof-of-principle detector setup. The amount of potassium contamination, determined by the direct counting of radioactive $^{40}$K, is found to be $2.2\pm1.5$ ppb, lowest ever achieved for NaI(Tl) crystals. With the active veto, the average background rate in the crystal in the 1-6 keV energy region-of-interest (ROI) is $1.20\pm0.05$ counts/day/kg/keV, which is a breakthrough since the DAMA/LIBRA experiment. 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. We discuss ongoing developments of the crystal manufacture aimed at the further reduction of the background, including data from purification by zone refining. A projected background rate lower than $\sim$0.2 counts/day/kg/keV in the ROI is within reach. These results represent a benchmark for the development of next-generation NaI(Tl) detector arrays for the direct detection of dark matter particles.