1. Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194
2. IMR, Tohoku University, Aoba-ku, Sendai 980-8577
3. Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195
4. Materials Science and Technology, University of Washington, Box 35210, WA 98196, U.S.A.