Status and future prospect of 48Ca double beta decay search in CANDLES

摘要: The observation of neutrino-less double beta decay (0vss) would be the most practical way to prove Majorana nature neutrino and lepton number violation. CANDLES studies 48Ca using CaF2 scintillator. main advantage is that it has highest Q-value (4.27 MeV) among all isotope candidates for 0vss. III detector currently operating with 300kg crystals in Kamioka underground observatory, Japan. In 2014, a cooling system magnetic cancellation coil was installed aim increase light emission scintillator photo-electron collection efficiency photo-multipliers. After this upgrade, yield increased 1000 p.e./MeV which 1.6 times larger than before. According data analysis simulation, background source turned out high energy external gamma-ray originating neutron capture on surrounding materials, so called (n,γ). Upgrading by installing shield can reduce remaining backgrounds two order magnitude. report, we discuss detail (n,γ) reduction additional shielding.