ZICOS - Neutrinoless double beta decay experiment using Zr-96 with an organic liquid scintillator -

A liquid scintillator containing a tetrakis (isopropyl acetoacetato) zirconium has been developed for ZICOS experiment. In order to reach the sensitivity T₁⁰_/^ν₂ ≥ 10²⁷ years, we have to use tone scale of ⁹⁶Zr and have to reduce 95 % of ²⁰⁸Tl decay background. Using Monte Carlo simulation, we could demonstrate new method using the topological information of Cherenkov light, and could reduce 93 % of ²⁰⁸Tl background with 78 % efficiency for 0νββ signal. For the discrimination of Cherenkov light, the precise spectral pulse shape from Cherenkov lights was directly measured by using sub-MeV electrons from ⁹⁰Sr/⁹⁰Y beta source. The observed shape was clearly different from the shape of scintillation light. The pulse rise and fall (decay) time for Cherenkov light were 0.8 ns and 2.5 ns, respectively. They were actually shorter than those times of scintillation light which were also measured by 1.6 ns and 6.5 ns, respectively. This clear difference of rise time will be used for the pulse shape discrimination in order to select PMT which receive Cherenkov lights, and the topological information due to Cherenkov light will be used for the reduction of backgrounds from ²⁰⁸Tl decay which should be major backgrounds observed around Q-value (3.35MeV) of ⁹⁶Zr neutrinoless double beta decay.