Fission-evaporation competition in Pu isotopes of mass 235-239

Abstract

Cross sections are reported for the reactions $^{233,234,235}\mathrm{U}(^{4}\mathrm{He},xn)$, for $x=1\ensuremath{-}4$. Comparisons are shown to calculations based on equilibrium and preequilibrium models. Fits of preequilibrium-model calculations to the tails of the excitation functions require significantly different parameters from those in the literature for other systems. There is a suggestion for emission of a second preequilibrium neutron. The equilibrium-model calculations show systematic deviations from the data that can be related to the empirically determined energy dependence of $\frac{{\ensuremath{\Gamma}}_{n}}{({\ensuremath{\Gamma}}_{n}+{\ensuremath{\Gamma}}_{f})}$. The fission barrier parameters for isotopes of Pu are refined. Their relationship to the excitation functions for ground state and shape isomeric nuclei is discussed.NUCLEAR REACTIONS, FISSION $\ensuremath{\sigma}(E)$ for $^{233,234,235}\mathrm{U}(^{4}\mathrm{He},xn)$ reactions ($x=1\ensuremath{-}4$) as competitors with fission; calculations with preequilibrium and equilibrium models for ${E}^{*}<~40$ MeV with double-humped fission barrier.