1. School of Health Science, Gunma University Faculty of Medicine, Maebashi 371-8514, Japan
2. 21st Century COE Program, Gunma University, Maebashi 371-8514, Japan
3. Advanced Radiation Technology Department, Japan Atomic Energy Agency, Takasaki 370-1292, Japan
4. South Product Co. Ltd, Uruma City 904-2234, Japan