1. Christie Hospital and Holt Radium Institute, Manchester; Mount Vernon Hospital, Northwood, Middlesex; Argonne National Laboratory. Illinois; E. I. du Pont de Nemours and Co., Wilmington, Delaware.
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3. Keene, J. P., Nature, 197, 47 (1963).
4. Baxendale, J. H., Fielden, E. M., and Keene, J. P., Proc. Chem. Soc., 242 (1963).
5. Keene, J. P., Rad. Res. (in the press).