KamLAND-PICO project to search for cosmic dark matter

KamLAND-PICO project aims to search for WIMPs dark matter by means of NaI(Tl) scintillator. To investigate the WIMPs candidate whose cross section is as small as ${10}^{-9}$ pb, a pure NaI(Tl) crystal was developed by chemical processing and taking care of surroundings. The concentration of U and Th chain was reduced to $5.4\pm 0.9$ ppt and $3.3\pm 2.2$ ppt, respectively. It should be remarked that the concentration of ${}^{210}$Pb which was difficult to reduce reached to the high purity as $58\pm 26$ $\mu$Bq/kg.