Thallium bromide ͑TlBr͒ is attractive for high energy radiation detection, given its large molecular weight and wide energy bandgap. However, TlBr exhibits levels of ionic conductivity that can lead to an undesirable leakage, or dark current, thereby reducing sensor performance. To investigate the role of dopants in controlling the ionic conductivity, single crystals of TlBr were grown using zone refining and/or vertical Bridgman methods with controlled levels of donor ͑Pb͒ dopants. Their electrical properties were examined as a function of temperature ͑20-300°C͒ with frequency dependent impedance spectroscopy. A Schottky-based defect equilibria model was fitted to the resulting conductivity data, and enthalpies of Schottky defect formation ͑0.91 Ϯ 0.03 eV͒, cation migration ͑0.51 Ϯ 0.03 eV͒, and anion migration ͑0.28 Ϯ 0.05 eV͒ were extracted. Br vacancies were found to posses about 5 orders of magnitude higher mobility than that of Tl vacancies at 20°C.
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±0.2 ppb, unprecedented for NaI(Tl) crystals. The 210 Pb activity is estimated to be 0.34±0.04 mBq/kg via α 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.
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