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발표제목: Single crystal scintillators for rare event search
This research aims to study the single crystal growth and characterization of inorganic scintillators for dark matter search, solar axion search and neutron detection. To fulfill the requirements of these applications, the scintillator must have high interaction cross-section for the candidate particles to be detected, high light yield, low background and a good match of emission spectrum wavelength with the spectral sensitivity curve of light sensor. This study describes the development of inorganic scintillators based on Li-contained materials.
The weakly interacting massive particles (WIMPs) are the most promising dark matter candidates. These WIMPs can be directly detected through the elastic scattering with a high spin factor nucleus present in a detector medium through their spin-dependent interaction. Therefore, Li- contained inorganic scintillators are attractive materials for WIMP search experiments due to the high spin factor of 7Li-nucleus. Another application of 7Li-contained scintillators is their use in solar axion search experiment. The 7Li nuclei in target crystal can be resonantly excited by hypothetical axions emitted in the de-excitation of the 7Li nuclei in the Sun. The γ-quanta emitted in the subsequent decay of 7Li can be detected by the Li-containing crystal scintillator. On the other hand, the presence of 6Li in the host lattice makes it favorable candidate for the detection of thermal neutrons.
Bridgman-Stockbarger and Czochralski are the two of the frequently used melt growth methods for single crystal scintillators. The Czochralski technique is mostly used to grow oxide and non-hygroscopic crystal scintillators. This technique can be easily implemented and provides the feasibility of growing large volume of single crystals. I have successfully grown Li2GeO3 crystals and LiF crystals with various dopants using the Czochralski method. The Bridgman-Stockbarger technique is well suited for the growth of hygroscopic crystals having low melting point. I have used this technique for the growth of LiI crystals with various dopants. Detailed description of starting material preparation and crystal growth procedures are presented.
After the growth process, grown crystals are characterized by measuring their luminescence and scintillation properties. X-ray and photo- induced luminescence of the grown crystals are measured at room temperature. Measurement of the scintillation properties include: light yield, energy resolution and scintillation decay time. The responses of the LiI(Tl) crystals for spallation neutrons were also measured at Korea multi-purpose accelerator complex (KOMAC). Moreover, I have measured the temperature dependence of light yield and scintillation decay time for LiF and LiI crystals from room temperature down to 10 K.
Comparison with the reported results in the literature show that our grown scintillation crystals have potentials to find their places in applications such as dark matter search, solar axion search and neutron detection.
Thesis Advisor: Prof. Hongjoo Kim