Rb and Cs Isotopic Cross Sections from 40-60-MeV-Proton Fission ofU238,Th232, andU235

The isotopic distributions of Rb and Cs from the fission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, and ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ induced by 40- to 60-MeV protons have been measured by means of an on-line mass spectrometer. The Rb isotopic distributions have a Gaussian shape, but those of Cs are somewhat asymmetrical. As the proton bombarding energy increases, the neutron-excess sides of the distributions remain approximately fixed while the neutron-deficient sides shift to lower mass numbers. The distributions also show significant variations with the neutron-to-proton ratio of the target. All the isotopic cross sections show a significant odd-even structure, with the formation of even neutron isotopes being favored. The effect is more pronounced for the neutron-rich isotopes. A similar structure is found to occur in Rb and Cs distributions from fission induced by thermal neutrons, 155-MeV protons, and 24-GeV protons, as well as in the Na and K cross sections from 24-GeV-proton reactions with a variety of targets. The odd-even effect in the Rb and Cs distributions can be accounted for by a 10 to 15% neutron-pairing effect in the prompt yields and a 2 to 3% pairing effect in the neutron emission. From the mean mass numbers of the Rb and Cs distributions, the average total number of emitted neutrons has been estimated for each reaction. This information, together with other results on neutron emission as a function of fragment mass, has allowed the mean mass numbers to be corrected for prompt neutron emission. These corrected values differ by about one mass unit from the values predicted by the unchanged-charge-density mechanism and are consistent with the same mechanism of charge division as that operating in thermal-neutron fission.