1. Intense Pulsed Neutron Source Division, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, Illinois 60439, Laboratoire Léon Brillouin (CEA-CNRS), CEA Saclay, 91191 Gif-sur-Yvette cedex, France, and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom