1. Division of Isotope Research and Development, Department of Research Reactor, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195, Japan
2. Faculty of Engineering, Aomori University, Aomori 030-0943, Japan
3. College of Engineering and Research Center of Ion Beam Technology, Hosei University, Koganei, Tokyo 184-8584, Japan