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 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.
Highlights
The nature of dark matter is one of the most important questions of modern physics
Most attempts at directly detecting dark matter have yielded null results, the DAMA/LIBRA experiment operating underground at the Laboratori Nazionali del Gran Sasso (LNGS) observes an annual modulation in event rate. Such a modulation is expected from the annual change in the relative velocity between the Earth and the dark matter halo as the Earth rotates around the Sun and the Sun moves at a constant speed in the dark matter halo [1,2]
6 kg of ultra-high purity NaI powder was thoroughly dried at Radiation Monitoring Devices (RMD) to remove trace amounts of water which can affect crystal growth adversely, and contribute to crystal radioactivity via 3H decay
Summary
Most attempts at directly detecting dark matter have yielded null results, the DAMA/LIBRA experiment operating underground at the Laboratori Nazionali del Gran Sasso (LNGS) observes an annual modulation in event rate. DAMA/LIBRA and its predecessor DAMA/NAI have been taking data for over 16 years, achieving a statistical significance of 12.9σ [2] for the annual modulation signal Such an annual modulation is in tension with the results of many other direct dark matter detection experiments [4,5,6]. Several experiments have been trying to confirm or refute the DAMA/LIBRA annual modulation, including ANAIS, DM-Ice, KIMS, and COSINE [7,8,9,10] None of these efforts have achieved a background low enough to confirm or refute the modulation effectively within a reasonable amount of time, and the biggest source of background comes from radioactive impurities inside the crystal. We report our progress in developing ultrahigh purity NaI(Tl) crystals, and present results of impurity measurements on NaI-033—our most recent 3.4-kg NaI(Tl) scintillating crystal
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