1. Okayama University, Faculty of Science, Okayama 700-8530, Japan
2. Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Kamioka, Gifu 506-1205
3. Institute for Environmental Sciences, Department of Radioecology, Aomori 039-3212, Japan
4. Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa, Chiba 277-8582, Japan
5. Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan