1. Institute of Industrial Science, The University of Tokyo, Komaba Meguro-ku, Tokyo 153-8 505, Japan
2. School of Engineering, Tokyo Denki University, Senju 5 Asahi-cho, Adachi-ku, Tokyo 120-8 551, Japan
3. Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1 195, Japan