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

The relative yields of the Rb-86-97 and Cs-130-144 isotopes in the U-2 38 fission induced by protons with energies E(p) = 70 and 90 MeV, resp ectively, are measured on a mass spectrometer operating on-line with a n accelerator. The contribution of direct reactions in uranium fission induced by protons with an energy of 90 MeV is determined by decompos ing the isotope distribution of fragments into two Gaussian curves and is found to be about 25%. It is shown that the centroids of some comp onents 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 concl uded that, in addition to the competition between two reaction mechani sms (compound nucleus and direct reactions), the formation of isotope distributions of fragments in nuclear fission by intermediate-energy p rotons is determined, to a certain extent, by shell effects.