1. Institute of Modern Physics, Shanxi Normal University, Linfen 041004, P. R. China
2. China Institute of Atomic Energy, P. O. Box 275 (96), Beijing 102413, P. R. China
3. Linfen Occupational Technology College, Linfen 041000, Shanxi, P. R. China
4. Radiation Measurement Research Section, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan