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

TL;DR

This paper reports the development of highly radiopure NaI(Tl) scintillators with significantly reduced radioactive impurities, enabling more sensitive dark matter detection experiments.

Contribution

The study introduces optimized methods for reducing radioactive impurities in NaI(Tl) crystals, achieving unprecedented radiopurity levels for dark matter searches.

Findings

Reduced $^{40}$K to 20 ppb in NaI(Tl) crystals.

Lowered thorium and uranium progenies to microbecquerel levels.

Achieved radiopurity levels suitable for dark matter detector construction.

Abstract

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