1. J. Orcel, B. David, F. Kraut, D. Nordemann, J. Tobailem, Compt. Rend., 264 (1967) 1556.
2. D. Nordemann, J. Tobailem, C. De Lassus, E. Barouch. Unpublished.
3. Y. Cantelaube, P. Pellas, in Origin and Distribution of the Elements, Ed. by L. H. Ahrens, Pergamon Press, London (1968) p. 479.
4. D. Lal, Communication personnelle.
5. K. Marti, J. P. Shedlovsky, R. M. Lindstrom, J. R. Arnold, N. G. Bhandari. This conference, Paper No. 22, p. 246.