SUMMARY The Problem Since fallout is a heterogeneous mixture of individual nuclides each of which has its own decay rate, an experimental determination of the composite decay rate fora fallout sample is necessary. Certain fallout samples were retained from Operation CASTLE and their decay rates measured, Findings Most of the values found for the exponent of theusual decay equation exceeded the value normally used in describ-. ing thermal-neutron fission of U238, iii UNCLASSIFIED ee eee eee ee