1. NHK Science and Technical Research Laboratories, 1‐10‐11, Kinuta, Setagaya‐ku, Tokyo 157, Japan
2. Sony Corporation Research Center, Hodogaya‐ku, Yokohama 240, Japan
3. Sumitomo Heavy Industries, Limited, Yato‐cho, Tanashi, Tokyo 188, Japan
4. National Laboratory for High Energy Physics, Photon Factory, 1‐1, Oho, Tsukuba, Ibaraki 305, Japan