The high purity NaI(Tl) scintillator has been developed to search for cosmic dark matter. The dark matter problem is one of the most important and attractive problem in cosmology and particle physics. Many groups are trying to develop highly sensitive detectors to search for dark matter candidates. PICO-LON group has developed a highly sensitive NaI(Tl) detector. The concentrations of ~(210)Pb,U-chain and Th-chain in the crystal were successfully reduced to 58±26 μBq/kg,5.4±0.9 ppt and 3.3±2.0 ppt, respectively. The concentrations in the NaI(Tl) developed by PICO-LON were comparable to the crystal developed by DAMA/LIBRA. The dark matter search project PICO-LON will give the significant test of the annual modulating signal of dark matter.
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