1. Institute of Modern Physics, Chinese Academy of Sciences , Lanzhou 730000, China
2. University of Chinese Academy of Sciences , Beijing 100049, China
3. Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University , Zhuhai 519082, China
4. SRON Netherlands Institute for Space Research , Niels Bohrweg 4, 2333 CA Leiden, the Netherlands