1. Amersham/Searle Corporation, Arlington Heights, Illinois, U.S.A.
2. L. Contreras, R. Gillis, G. Kyle, J. Spencer and S. Vigdor, High resolution beam scanner, Los Alamos, Internal Report.
3. F. Karacek, Argonne National Laboratory, Argonne, Illinois, U.S.A.
4. Pfizer, Inc.; Minerals, Pigments and Metals Division, Los Angeles, California, U.S.A.