1. Geochemical Research Center, Graduate School of Science The University of Tokyo 7‐3‐1 Hongo, Bunkyo‐ku Tokyo Japan
2. Atmosphere and Ocean Research Institute The University of Tokyo 5‐1‐5 Kashiwanoha, Kashiwa‐shi Chiba Japan
3. Kyoto Fission‐Track Co. 44‐4 Minamitajiri‐cho, Omiya, Kita‐ku Kyoto Japan