1. Department of Electronic Chemistry, Tokyo Institute of Technology, Nagatsuta 4259, Midori‐ku, Yokohama 226, Japan
2. Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2‐12‐1, O‐okayama, Meguro‐ku, Tokyo 152, Japan
3. Department of Materials Science, Faculty of Engineering, Chiba University, Yayoi‐cho, 1‐33, Inage‐ku, Chiba 263, Japan