Development of highly radiopure NaI(Tl) scintillator for PICOLON dark matter search project

Abstract Highly radiopure NaI(Tl) was developed to search for particle candidates of dark matter. Optimized methods were combined reduce various radioactive impurities. $^{40}$K effectively reduced by the recrystallization method. The progenies decay chains uranium and thorium appropriate resins. concentration natural potassium in crystal 20 ppb. Concentrations alpha-ray emitters successfully resin selection. present concentrations series $^{226}$Ra $1.2\pm1.4$$\mu$Bq/kg $13\pm4$$\mu$Bq/kg, respectively. No significant excess $^{210}$Pb obtained, upper limit 5.7 $\mu$Bq/kg at 90$\#$ CL. achieved level radiopurity crystals makes construction a matter detector possible.