becserrtorREGTEDIDATA C 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 cither 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 elements formed initially partially separat with respect to /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 radiochemi- cal 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