Study of 48Ca Double Beta Decay

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The observation of double beta decay demonstrates the violation of Lepton Number conservation. It is a key experiment to answer the question why our universe is matter dominated. This research project aims at studying the double beta decay of 48Ca for which the CANDLES detector system, which features CaF2 crystals, is under construction. This year is the final year of the research. It is fortunate that our request to continue the research approved and our final year becomes the starting year of the basic Research S which roughly doubles our budget.We have achieved the following this year.1. We constructed vessel and oil purification system. We now have almost all elements of the detector.2. Newly designed PMT's are immersed in pire water and endurance test was performed. We found some problem and fixed it.3. We studied best arrangement of PMT for given number of PMT's.4. Additional CaF2 crystals were purchased. We studied radioactive background level and found that all of them are OK.5 … More . The almost unique background which is serious to this experiment is the successive decay of beta and alpha particles that may give background at the Q value region. The observation of pulse shape will give us information to discriminate the background for which we studied characteristics of 500 MHz flash ADC.6. Although we are working at Oto cosmo observatory near Osaka University, the CANDLES detector system becomes so large that Oto will have difficulty to accommodate the detector. We had discussion with ICRR and we decided to install the CANDLES III at Kamioka underground laboratory.7. We organized US-Japan seminar on "Double beta decay and neutrino masses" at Maui Hawaii from September 16-20, 2005. In this seminar we discussed current status of study of double beta decay. It helped to have recognition of our experiment.We are currently studying characteristic of the detector we constructed at Osaka University. After that study we will move the detector system to underground laboratory and start data taking. Less