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 $5.4\pm0.9$ ppt and $3.3\pm2.2$ ppt, respectively. It should be remarked that the concentration of $^{210}$Pb which was difficult to reduce reached to the high purity as $58\pm26$ $\mu$Bq/kg. 
K.Fushimi, Y.Awatani, H.Ejiri, R.Hazama, H.Ikeda, K.Imagawa, K.Inoue,   A.Kozlov, R.Orito, T.Shima, R.Sugawaraand K.Yasuda
astro-ph.IM physics.ins-det