1. Japan Atomic Energy Research Institute, Naka Fusion Research Establishment 801 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken 311-0193, Japan
2. Japan Atomic Energy Research Institute, Advanced Radiation Technology Center Takasaki Radiation Chemistry Research Establishment, 1233 Watanuki Takasaki-shi, Gunma-ken 370-1292, Japan