1. Research Reactor Institute, Kyoto University, 2 Asashironishi, Kumatori, Sennan, Osaka 590-0494, Japan
2. The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
3. Superabsorbents Research Department, Nippon Shokubai Co., Ltd., 992-1 Aza Nishioki Okihama, Aboshi-ku, Himeji, Hyogo 671-1292, Japan