1. Research Institute for Measurement and Analytical Instrumentation (RIMA), National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
2. Materials Science Research Center (MSRC), Japan Atomic Energy Agency (JAEA), Tokai, Naka, Ibaraki 319-1195, Japan