Abstract:

Study of neutrino-less double beta (0nbb) decay becomes of particular importance after the confirmation of neutrino oscillation which shows that neutrinos have mass. They could then be Majorana particles which violate lepton number conservation. Once lepton number non-conservation is verified, we have a scenario to explain how our matter dominated universe is realized dynamically. It is the leptogenesis combined with CP violation in lepton sector. 0nbb decay is the only known process to verify the Majorana nature of neutrinos. We have developed the CANDLES detector system for the study of 48-Ca 0nbb decay. The 48-Ca has the highest Q value (4.3 MeV) among double beta decay nuclei. It means that the large decay rate for a given neutrino mass and the least background. CANDLES employs CaF2 crystals for the double beta source and the detector. They are immersed in liquid scintillator in an acrylic vessel. Scintillation signals are viewed by PMTâ™s surrounding the vessel. The design achieved effective background rejection by using decay time difference of CaF2 (~1 micro sec) and liquid scintillator (~10 nano sec). We constructed a prototype detector at sea level laboratory which has 200 kg CaF2 crystalsi, and it is now installed at the Kamioka underground laboratory. I would like to describe why we believe that the neutrino has to be a Majorana particle and the present and future of the CANDLES project.