1. State Key Laboratory for Particle Detection and Electronics, Hefei 230026, P. R. China
2. Department of Modern Physics, University of Science and Technology of China, Hefei 230026, P. R. China
3. Experimental Particle Physics Group, University of Chinese Academy of Science, Beijing 100049, P. R. China
4. Center for Underground Physics, Institute for Basic Science, Daejeon 34126, South Korea