Spallation-Fission Competition in Heaviest Elements; Helium-Ion-Induced Reactions in Uranium Isotopes

Abstract

A radiochemical study of fission and spallation products produced by bombardment of ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{U}}^{238}$ with 18-46 Mev helium ions has been made. As in the case of similar studies using isotopes of plutonium as targets, most of the reaction cross section is taken up by fission. Also, the pronounced increase of the total cross section for ($\ensuremath{\alpha}, \mathrm{xn}$) reactions with increasing mass number of the target that was observed for plutonium targets is observed for uranium targets.Excitation functions for ($\ensuremath{\alpha}, 2n$), ($\ensuremath{\alpha}, 3n$), and ($\ensuremath{\alpha}, 4n$) reactions are interpreted in terms of compound-nucleus formation and fission competition at the various stages of the neutron evaporation chain. The importance of neutron binding energies for the competition between fission and neutron emission is stressed. An existing model for neutron evaporation following compound-nucleus formation has been extended to include the effect of fission competition. Results of calculations based on this model show good agreement with those features of the ($\ensuremath{\alpha}, \mathrm{xn}$) excitation functions believed to result from compound-nucleus formation. These calculations also show that fission usually precedes neutron evaporation for helium-ion-induced reactions of ${\mathrm{U}}^{233}$ and ${\mathrm{U}}^{235}$. The excitation functions for the ($\ensuremath{\alpha}, n$), ($\ensuremath{\alpha}, p$), ($\ensuremath{\alpha}, pn+\ensuremath{\alpha}, d$), ($\ensuremath{\alpha}, p2n+\ensuremath{\alpha}, t$), and ($\ensuremath{\alpha}, p3n+\ensuremath{\alpha}, \mathrm{tn}$) reactions are discussed in terms of direct interaction mechanisms involving little competition from fission.Fission shows an increase in symmetry with energy and becomes symmetric at about 40-Mev energy of the helium ions. There is no significant difference in the symmetry of fission for the three uranium isotopes. Total reaction cross sections, including those for both fission and spallation reactions, indicate a nuclear radius parameter ${r}_{0}$ slightly larger than 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm.