1. National Institute of Radiological Sciences, 9-1, Anagawa-4-chome, Chiba 260 Japan
2. University of Tokyo, Institute for Nuclear Study, Midori-cho, 3-2-1, Tanashi, Tokyo 188 Japan
3. Kyoto University, Department of Nuclear Engineering, Yoshida-Honmachi, Sakyo, Kyoto 606 Japan