1. Department of Earth and Planetary Sciences; Hokkaido University; Sapporo 060-0810 Japan
2. JEOL Ltd.; Akishima 196-8558 Japan
3. Department of Physics; Osaka University; Toyonaka 560-0043 Japan
4. Graduate School of Engineering Sciences; Kyushu University; Kasuga 816-8580 Japan
5. National Institute for Materials Science; Tsukuba 305-0044 Japan
6. Institute of Space and Astronautical Science; Japan Aerospace Exploration Agency; Sagamihara 252-5210 Japan