1. School of Nuclear Science and Technology, University of South China, Hengyang, Hunan 421001, People's Republic of China
2. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, People's Republic of China
3. School of Environmental Protection and Safety Engineering, University of South China, Hengyang, Hunan 421001, People's Republic of China
4. School of Mathematics and Physics, University of South China, Hengyang, Hunan 421001, People's Republic of China