1. Hydrogen Isotope Research Center, University of Toyama, 3190 Gofuku, Toyama-shi, Toyama Pref. 930-8555, Japan
2. Tritium Technology Group, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Tokai-mura, Naka-gun, Ibaraki Pref. 319-1195, Japan
3. Institute for Material Research, Tohoku University, 2145-2 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Pref. 311-1313, Japan