Majorana neutrinos studied by double beta decays and MOON

High sensitivity experiments of neutrino-less beta beta decays (0 nu beta beta) are of great interest for studying the Majorana nature of nu's, the nu mass spectrum, the absolute nu mass scale, the Majorana CP phases and others beyond the standard model (SM). Spectroscopic studies for energy and angular correlations of two beta rays are indispensable for establishing the 0 nu beta beta and the Majorana mass. MOON is a spectroscopic double beta decay (beta beta) experiment with a neutrino-mass sensitivity in the quasi-degenerate and inverted hierarchy (QD-IH) mass region (100-30 meV). It has a multi-layer structure of plastic scintillator plates to accommodate a ton-scale double beta source, a good energy resolution of sigma approximate to 2.5 % at the 0 nu beta beta peak energy, and a good background rejection capability. A prototype detector MOON-1 was built to demonstrate the feasibility and the sensitivity of MOON. The scintillation photon collection and the energy resolution are found to be as good as required for the mass sensitivity.