1. College of Physics and Electronic Science, Shanxi Datong University, Datong 037009, P. R. China
2. Department of Nuclear Physics, China Institute of Atomic Energy, P. O. Box (275-41), Beijing 102413, P. R. China
3. Institute of Applied Physics and Computational Mathematics, Beijing 100094, P. R. China
4. Department of Physics, Nankai University, Tianjin 300071, P. R. China