1. Neutron Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801
2. Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
3. ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxon, 0X11 OQX, U.K.