1. School of Physics and Electronic Science, Guizhou Education University, Guiyang 550018, P. R. China
2. Department of Mathematics and Physics, North China Electric Power University, Baoding 071003, P. R. China
3. China Institute of Atomic Energy, Beijing 102413, P. R. China
4. Guangxi Key Laboratory of Nuclear, Physics and Technology, Guangxi Normal University, Guilin 541006, P. R. China