1. Radioisotope Research Center, Kyoto University, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
2. Forestry and Forest Products Research Institute, P. O. Box 16, Tsukuba Norin Kenkyu Danchi-nai, Ibaraki 305-8687, Japan
3. Department of Nuclear Engineering, Kyoto University, Uji, Kyoto 611-0011, Japan
4. Graduate School of Agriculture, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan