Symmetric and Asymmetric Fission

Abstract

Fission yields are calculated assuming as a first approximation that they are proportional to the product of the level densities of a pair of binary fission products. The level densities are calculated with simplified shellmodel methods. The calculations predict a single symmetric peak when the mass of the fissioning nucleus, A/sub 0/, is less than 210 (in agreement with the observed fission yields for bismuth and lighter alements), and three msxima in the fission yield curves for heavier compound nuclei. The peak corresponding to approximately equal-size binary fission products is very much higher than is observed experimentally. This is undoubtedly due to the fact that in asymmetric fission a core corresponding to 82 neutrons and 50 protons remains intact in the heavier fission prod uct, whereas for symmetric fission this core is disrupted at the cost of several Mev. Since correction for this energy effect involves a number of unknown factors, the calculated yields for symmetric fission are reduced by the same empirical factor in all calculations. An additional parameter, n, is introduced in correcting for excitation energy of the fission products and for possible departhres from equilibrium. The correction calculations, which involve only two free parameters, explain most ofmore » the fission yield data for all five known cases (Pu/sup 2//sup 3//sup 9/, U/sup 2//sup 3//sup 8/, U/sup 2//sup 3//sup 5/, U/sup 2//sup 3//sup 3/, and Th/sup 2//sup 3//sup 2/) where the compound nucleus is within a Mev or so of the fission threshold, but it is necessary to treat n as a free parameter for each curve to fit the small steep regions on each side of mass number 1/2A/sub 0/. The calculated fission yields of the more highly excited compound nucleus, Ac/sup 2//sup 2//sup 7/, predict three equally prominent maxima in qualitative agreement with observation. (auth)« less