Growth of Ultra-high Purity NaI(Tl) Crystal for Dark Matter Searches

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 radio-purity. 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 $^{39}$K concentration in the final crystal is estimated to be 4.3$\pm$0.2~ppb, unprecedented for NaI(Tl) crystals. The $^{210}$Pb activity is estimated to be 0.34$\pm$0.04~mBq/kg via $\alpha$ counting of $^{210}$Po, 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 higher purity NaI(Tl) crystals for dark matter searches.