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Crystal scintillators are widely used for searching dark matter (Weakly Interacting Massive Particles). DAMA, ANAIS, and COSINE experiment are using NaI:Tl crystal while KIMS experiment is CsI:Tl crystal for WIMP dark matter search, respectively. To improve the sensitivity of these experiment, a scintillator that has a higher light yield is required. Since pure CsI and NaI crystals have 2 times higher light yields than CsI:Tl and NaI:Tl at low temperatures, these crystals can be good candidates for low-temperature dark matter search experiments. Thus, we studied the characteristics of pure CsI and NaI scintillators at low temperatures to check their feasibility for dark matter search. The NaI crystal used in this experiment was grown from high purity Astro-grade (99.999%, ultra-low potassium) NaI powders by Czochralski method in platinum crucible in argon atmosphere. X-ray-induced luminescence was measured at room temperature to confirm that the crystals used in this experiments are high purity CsI and NaI crystals. The 5.5 MeV α-particles from 241Am and 662 keV γ-rays from 137Cs sources were used to measure the light yield, α/β ratio, and decay time for each temperature, and by using pulse shape discrimination technique, it is confirmed if the α-particle and γ-ray signals are well distinguished. In this research, we studied the temperature dependence of CsI and NaI crystals.
Thesis Advisor: Prof. Hongjoo Kim