Dark matter search project PICO-LON

Understand

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.