1. Toki Geochronology Research Laboratory, Tono Geoscience Center, Japan Atomic Energy Agency, 959-31 Jorinji, Izumi, Toki, Gifu 509-5102, Japan
2. Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aramaki-Aza-Aoba, Aoba, Sendai 980-8579, Japan
3. Institute of Geosciences, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
4. Graduate School of Environmental Studies, Nagoya University, Furo-cho, Nagoya 464-8602, Japan