1. Textile Research Institute of Gunma, 5‐46‐1 Aioi‐cho, Kiryu, Gunma 376‐0011, Japan
2. Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, 1233 Watanuki‐machi, Takasaki, Gunma 370‐1292, Japan; Fax: +81‐27‐346‐9694