Radiochemical Study of the Ranges in Metallic Uranium of the Fragments from Thermal Neutron Fission

Abstract

The mean ranges in uranium of 28 fission products have been determined by radiochemical measurement of the fraction escaping from the surface. Several factors affecting the precision and accuracy of the method are discussed. A semiempirical equation was developed which gave an excellent correlation between the ranges of fragments from a specific mass chain and their average initial velocity. The average total kinetic energy of the fragment pairs is about 30 Mev less for the products of symmetrical fission than is expected from comparison with the asymmetrical products. The low momenta of the symmetrical fragments are not readily explained by particle emission unless the concept of isotropic neutron evaporation is abandoned. The results may be interpreted by assuming that the symmetrical and asymmetrical products result from two modes of fission which involve different critical shapes of the fissioning nucleus, and that the choice between modes is dependent on the closing of the 50-proton shell in the heavy fragment. The 10% decrease in range observed for two shielded nuclides is also examined in some detail.