Growth of ultra-high purity NaI(Tl) crystals for dark matter searches

Abstract

The annual modulation of scintillation event rate observed by the DAMA/LIBRA experiment has been a long-standing controversy in the quest of the direct detection of dark matter. The effort to definitively confirm or refute the annual modulation has turned out to be challenging due to the lack of NaI(Tl) crystals with high enough radiopurity. Most recently, we successfully grew a 6-kg ingot free from contamination during growth, from which a 3.4-kg crystal scintillator was made. The K39 concentration in the final crystal is estimated to be 4.3±0.2ppb, unprecedented for NaI(Tl) crystals. The Pb210 activity is estimated to be 0.34±0.04mBq/kg via α counting of Po210, among the lowest of currently running NaI-based dark matter experiments except DAMA/LIBRA. More importantly, the techniques and protocols we have developed will further contribute to the growth of ultra-high purity NaI(Tl) crystals for dark matter searches.