The Observation of neutrinoless double beta decay is very important because
it establishes lepton number non-conservation and the Majorana nature of
neutrinos. We have been studying double beta decay of 48Ca with CANDLES
system. In CANDLES system, many CaF2s are immersed in liquid scintillator
acts as an active veto, and these are viewed by large PMTs. We constructed
CANDLES III detector with 191kg CaF2 crystals at sea level (Osaka
university).
First, the performance check of CANDLES III was carried out with respect to
the gain stability, the energy resolution and the ability of identification
of event points.
Next, The long time measurement was carried out. After the measurement, 220
events/191kg/hour as backgrounds of neutrinoless double beta decay was
obtained with live time 196hours.
I assume the backgrounds are arise from cosmic rays, and a simulation of
cosmic rays was carried out. But background rate of the simulation is about
half of that of the experiment. According to the simulation, gamma rays
generated by some precesses such as bremsstrahlung create the backgrounds.
Finally, I introduced the plan to move CANDLES III to the underground
laboratory.
Then other rejection methods for the backgrounds and measures against gamma
rays were proposed.