1. College of Physics, Sichuan University, Chengdu610064, P. R. China
2. Faculty of Engineering, University of Nottingham, University Park, Nottingham999020, U.K.
3. Key Laboratory of Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu610064, P. R. China
4. Nuclear Power Institute of China, Chengdu, Sichuan610041, P. R. China
5. School of Materials Science & Engineering, Sichuan University, Chengdu610064, China
6. Sichuan Research Center of New Materials, Institute of Chemical Materials, China Academy of Engineering Physics, 596 Yinhe Road, Shuangliu, Chengdu610200, P. R. China