0018933 The isobaric radionuclide chains formed in the explosion are known to be distributed on a mass scale in a way generally similar to the products of asymmetric fission of y235 by thermal neutrons, but with some important differences, The experimental yield curve for slow neutron fission has a broad minimum for mass numbers approximately one-half that of the original nucleus and maxima on either side at mass numbers in the neighborhood of 95 and 139 (Reference 7}, Comparing the chain yields for megaton-range detonations with this curve, it is noted that there is a small drop in the peak yields accompanied by an increase in the symmetric fission probability. The same nuclide distribution might be ex- , pected in the fallout material and this is found to be roughly true under certain conditions. In other cases, the chains formed initially separate from one another so that samples of fallout may differ in composition among themselves and also from the distribution curve characteristic for the event. Fractionation is a term which has been applied to this phenomenon and it is used to signify an alteration in nuclide composition of some portion of the debris which renders it non-representative of the bomb products as a whole. The R-values, which are commonly used for reporting radiochemical data on cloud and fallout samples, are useful indices of fractionation. The R-value for any nuclide is defined as the ratio of the number of atoms SAN BRUNO FRE 16