1. Present Address: Unidad de Investigatión Aplicada, División de Aplicaciones Energéticas, Instituto National de Investigaciones Nucleares, A.P. 18-1027, México 11801, D.F.
2. Present Address: Department of Chemistry, 108 Browne Hall, Syracuse University, Syracuse, New York 13244-1200
3. McGervey, J. D. 1967.“Positron Annihilation;”, Edited by: Stewart, A. T. and Roelling, L. O. 143New York: Academic Press.