1. Tohoku University, Cyclotron and Radioisotope Center, Aoba, Aramaki, Aoba-ku, Sendai 980, Japan
2. Institute of Physical and Chemical Research, Hirosawa, Wako, Saitama 351-01, Japan
3. University of Tokyo, Institute for Nuclear Study, 3-2-1, Midori-cho, Tanashi, Tokyo 188, Japan
4. Japan Atomic Energy Research Institute, Takasaki Research Establishment 1233 Watanuki-cho, Takasaki, Gunma 370-12, Japan