1. School of Nuclear Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, P. R. China
2. Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Aramaki-Aza-Aoba 6-6, Sendai, Miyagi 980-8579, Japan
3. UNION SHOWA K.K., 1-8-40 Kounan Minato-ku, Tokyo 108-0075, Japan