Table Date 4. Average Annual Americium Values 24lam x 1076 atoms/SCM 2414m/239pu atom ratio 1963 5.92 0.0011 1964 2.68 0.0012 1965 1.91 0.0023 1966 1.00 0.0025 1967 0.62 0.0027 1968 0.49 0.0012 1969 0.33 0.0014 1970 0.27 0.0007 266 indicates that ingrowth in 1967 with an apparent half-life of 1.6 years. This atmosphere. From from decay of 241py ig a negligible source of 24lam in the the injections of 1967-1970, the 241am/239Pu atom ratio decreased because . plutonium contained relatively smaller quantities of americium ACKNOWLEDGMENTS V. Hamilton of the Thanks are given ta SSgt Thomas W. Nibarger and Howard separating and purifying Chemistry Section of McClellan Central Laboratory for Special thanks are given to the plutonium samples reported in this study. this manuscript. Mrs. Marcene Beckinger for her help in the preparation of 267