PICOLON (Pure Inorganic Crystal Observatory for LOw-energy Neutr(al)ino) project is developing high-purity NaI(Tl) crystals to investigate the origin of dark matter.
The XENON experiment has widely excluded the existence of WIMPs.
However, the annual modulation of the signal reported by the DAMA/LIBRA group has not been rejected using the same target, NaI(Tl).
Recently, COSINE-100 reported an annually modulating signal of the opposite phase from the data taken using a 106 kg NaI(Tl) DM detector.
We plan to build a radiopure large-volume NaI(Tl) detector to resolve existing observation uncertainty.
We have developed and confirmed methods to remove radioactive impurities in NaI(Tl) crystals.
The concentration of natural potassium was less than 20 ppb, and the U, Th series were less than a few ppt.
Using large detectors effectively reduces background events originating from outside of the crystal.
The possibility of background reduction and prospects for cosmic dark matter searches with large NaI(Tl) crystals
using ultra-high purification techniques will be presented.