1. Faculty of Engineering Science, Osaka University, Toyonaka 1-3, Osaka 560-8531, Japan
2. Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan
3. Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki-shi 370-1292, Japan
4. The Institute of Physical and Chemical Research, Wako-shi, Saitama 351-0106, Japan
5. Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-0962, Japan