ISOTOPE EFFECTS IN FRAGMENT YIELDS IN URANIUM NUCLEAR-FISSION INDUCED BY INTERMEDIATE-ENERGY PROTONS

The relative yields of the Rb-86-97 and Cs-130-144 isotopes in the U-238 fission induced by protons with energies E(p) = 70 and 90 MeV, respectively, are measured on a mass spectrometer operating on-line with an accelerator. The contribution of direct reactions in uranium fission induced by protons with an energy of 90 MeV is determined by decomposing the isotope distribution of fragments into two Gaussian curves and is found to be about 25%. It is shown that the centroids of some components of this decomposition correspond to filled neutron shells with N = 56 (Rb) and 82 (Cs). The magnitude of the even-odd effect is found to increase in the same regions of isotope distributions. It is concluded that, in addition to the competition between two reaction mechanisms (compound nucleus and direct reactions), the formation of isotope distributions of fragments in nuclear fission by intermediate-energy protons is determined, to a certain extent, by shell effects.