1. Department of Physics, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong
2. Division of Radiation Protection and Safety Control, Cyclotron and Radioisotope Center, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
3. Graduate School of Biomedical Engineering, Tohoku University, Sendai 980-8579, Japan