1. Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka, 590-04 Japan
2. Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606, Japan
3. Department of Mechanical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, 560 Japan
4. The Institute of Physical and Chemical Research, Wako-shi, Saitama, 350-01, Japan