1. Department of Physics, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223
2. Booster Synchrotron Utilization Facility, National Laboratory for High Energy Physics, Oho 1-1, Tsukuba 305
3. ISIS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, U.K.