-3differential counting, by chemical separation of daughter products, or by some other appropriate method. We shall consider here a method based on the separation of a group of isobars. The average number of beta decays associated with a given fission fragment is about three or four.> Assuming that fission products more than eight beta decays away from stability are not formed in appreciable yield, one predicts half-lives of the order of 0.1 second or greater for the primary products, using a semi-empirical mass equation such as that of Levy’ beta-decay theory. with standard Thus the time from scission to complete passage through a mass spectrometer can be kept short enough so that no appreciable beta decay will occur. Since all mass separations operate on the ratio of mass to ionic charge, ambiguities in observed mass can result unless the fission fragments have a defined ionic charge, e.g., a mass-to-charge ratio of 25 could result from unresolved masses of 75, ' respectively. 100, 125, and 150 with charges of 3, 4, 5, and 6 Only species of a single charge give mass to charge ratios that are unambiguous over nearly the entire mass range (Fig. 1). Lassen” has shown that the ionic charges on fission fragments originating in a thin source range from 16 to 28; one would therefore expect considerable ambiguity in the interpretation of most charge-to-mass ratios. Some of this ambiguity could be resolved by constructing a high-resolution machine; however, it seems advisable either to search for ¢ means of reducing the spread in the values of the ionic charges, or to attempt to develop a system having at least an appreciable abundance of singly-charged species. The second alternative seems more reasonable to attempt, since we as- sume that interaction with matter will reduce the charge as well as the energy 5001832