Development of the Ultra-lav Backgnund Drerctor for neutrinc and clark matter study

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We have developed a CaFィイD22ィエD2 scintillation detector system (ELEGANTVI) to search for neutrino-less double beta decay of ィイD148ィエD1Ca and spin coupled dark matter by elastic scattering of ィイD119ィエD1F. The whole system is in operation at the underground laboratory located in Nara (Oto Cosmo Observatory). To improve the sensitivity of the detector system, we must increase the number of target nuclei. As a result, The size of the detector system becomes larger which brings the complexity and large costs. To suppress these, we employ a new design of the detector system which consists of CaFィイD22ィエD2 (pure) crystals surrounded by a liquid scintillator as an active 4π-shield.We constructed a test bench system to study the characteristics of this system for future larger detector. One or more CaFィイD22ィエD2 (pure) crystal (s) surrounded by a liquid scintillator are viewed by 4 (5")PMTs. The best value of the concentration of a scintillating solute was obtained by measuring the light yield with varying the concentration. By measuring the decay time of pulses, we can discriminate the signal from the CaFィイD22ィエD2 (pure) scintillator and from the liquid scintillator.We are now studying the best method to discriminate signal from noise by using test bench system.