1. School of Science, Southwest University of Science and Technology, Mianyang 621010, P. R. China
2. Key Laboratory for Neutron Physics, Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, P. R. China
3. Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, P. R. China
4. Department of Basic Education, Qinghai University, Xining 810016, P. R. China
5. School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan 430074, P. R. China