High purity NaI(Tl) scintillator to search for dark matter

TL;DR

This paper reports the development of a high purity, large volume NaI(Tl) scintillator with ultra-low radioactive impurities, aiming to enhance the sensitivity of dark matter detection experiments.

Contribution

The authors developed a novel high purity NaI(Tl) scintillator with significantly reduced radioactive impurities, improving dark matter search sensitivity.

Findings

Impurities like $^{226}$Ra reduced to <100 μBq/kg.

$^{210}$Pb impurity reduced to <30 μBq/kg.

Sensitivity to WIMPs estimated at 7×10^{-45} cm^2 for 50 GeV/c^2 mass.

Abstract

A high purity and large volume NaI(Tl) scintillator was developed to search for cosmic dark matter. The required densities of radioactive impurities (RIs) such as U-chain, Th-chain are less than a few ppt to establish high sensitivity to dark matter. The impurity of RIs were effectively reduced by selecting raw materials of crucible and by performing chemical reduction of lead ion in NaI raw powder. The impurity of $^{226}$Ra was reduced less than 100 $\mu$Bq/kg in NaI(Tl) crystal. It should be remarked that the impurity of $^{210}$Pb, which is difficult to reduce, is effectively reduced by chemical processing of NaI raw powder down to less than 30 $\mu$Bq/kg. The expected sensitivity to cosmic dark matter by using 250 kg of the high purity and large volume NaI(Tl) scintillator (PICO-LON; Pure Inorganic Crystal Observatory for LOw-background Neutr(al)ino) is 7$\times$10$^{-45}$ cm$^{2}$ for 50 GeV$/c^{2}$ WIMPs.