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 purified 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 ± 4 µBq/kg and 1.5 ± 1.9 µBq/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 ± 2 µBq/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.

The highly radiopure NaI(Tl) was developed to search for particle candidates of dark matter. The optimized methods were combined to reduce various radioactive impurities. The 40K was effectively reduced by the re-crystallization method. The progenies of the decay chains of uranium and thorium were reduced by appropriate resins. The concentration of natural potassium in NaI(Tl) crystal was reduced down to 20 ppb. Concentrations of alpha-ray emitters were successfully reduced by appropriate selection of resin. The present concentration of thorium series and 226Ra were 1:2 ± 1:4 μBq/kg and 13 ± 4 μBq/kg, respectively. No significant excess in the concentration of 210Pb was obtained, and the upper limit was 5.7 μBq/kg at 90% C. L. The achieved level of radiopurity of NaI(Tl) crystals makes construction of a dark matter detector possible.

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