Dark matter search project PICO-LON

TL;DR

The PICO-LON project develops highly radio-pure NaI(Tl) scintillators to improve the sensitivity of cold dark matter searches, achieving significantly reduced radioactive impurities and low background rates.

Contribution

This work demonstrates effective purification techniques for NaI(Tl) crystals, lowering radioactive impurities and background noise for dark matter detection.

Findings

Radioactive impurities of $^{226}$Ra and $^{228}$Th were reduced to micro-Becquerel levels.

The concentration of $^{210}$Pb was lowered to 24 μBq/kg.

The background rate at 10 keVee was as low as 8 counts/(keV·kg·day).

Abstract

The PICO-LON project aims at search for cold dark matter by means of highly radio-pure and large volume NaI(Tl) scintillator. The NaI powder was purifed by chemical processing to remove lead isotopes and selecting a high purity graphite crucible. The concentrations of radioactive impurities of $^{226}$Ra and $^{228}$Th were effectively reduced to 58$\pm$4 $\mu$Bq/kg and 1.5$\pm$1.9 $\mu$Bq/kg, respectively. It should be remarked that the concentration of $^{210}$Pb, which is crucial for the sensitivity to dark matter, was reduced to 24$\pm$2 $\mu$Bq/kg. The total background rate at 10 keVee was as low as 8 keV$^{-1}$kg$^{-1}$day$^{-1}$, which was sufficiently low to search for dark matter. Further purification of NaI(Tl) ingot and future prospect of PICO-LON project is discussed.