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.

Direct search for dark matter is one of the most important problems in astrophysics. Significant signal for dark matter will be a hint to clarify the origin of the universe. Only DAMA/LIBRA experiment with NaI(Tl) detector has ever suggested the presence of dark matter signal. Verifying the DAMA/LIBRA result by a NaI(Tl) detector is urgent and important task. We have tried to purify NaI(Tl) crystal to search for dark matter. In this presentation, the present status of purification will be discussed. The concentration of potassium is successfully reduced to desired sensitivity. The 210Pb, which is difficult to reduce, has been reduced effectively. Present status of low background measurement in Kamioka observatory will be shown.

The PICOLON project is developing a high purity NaI(Tl) crystal by optimizing the purification methods. The reduction method for 40K was established and we got sufficiently pure NaI(Tl) crystal for 40K. On the other hand, the concentration of 210Pb is still higher than our goal. The investigation is now in progress to fix the origin of the contamination. The construction of the second phase detector system of PICOLON detector will be started in 2020 spring. The total mass of the second phase is about 54 kg. The background level by the test measurement using a single module is reported.