1. Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong, China
2. Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstr., 400, 01328, Dresden, Germany
3. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstr., 400, 01328, Dresden, Germany