R&D of new high-performance scintillator including double beta decay nuclei

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Inorganic scintillators have been used for underground astroparticle physics experiment, such as dark matter search and double beta decay search. We have developed CaI 2 which has a large amount of luminescence and contains 48Ca from double beta decay nuclei. Half-inch CaI 2 crystals were prepared by the Bridgman method in Institute for Materia Research, cut into several mm squares, and the scintillation characteristics were measured. The light emission amount was 107,000 ph. / MeV, 2.7 times NaI (Tl) and 10 times undoped CaF 2 . The light emission wavelength was 410 nm, which was in agreement with sensitive wavelength of photomultiplier tubes. When the waveform discrimination characteristics were measured using alpha rays of 241Am and gamma rays of 137Cs, it was found that there was a large difference in the first 300 ns between the waveforms. Our pulse shape discrimination analysis using fast/total ratio method gives 4.2 for CaI 2 in figure of merit, and it was 2.0 for CaF2(Eu). CaI 2 showed high potential for pulse shape discrimination. Low temperature dependence of light yield was also investigated, and we confirm that light yield of CaI 2 increase at low temperature. At present, the growth and processing of large crystals is difficult due to the strong deliquescence and cleavage of CaI 2 . Work is underway to develop better growth and processing method for CaI 2 crystal.