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 .