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

Cross sections are reported for the reactions ${}_{}{}^{233,234,235}{}_{}{}^{}\mathrm{U}({}_{}{}^4{}_{}{}^{}\mathrm{He},xn)$, for $x=1\mathrm{}-4$. 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 $\frac{{\Gamma }_n}{({\Gamma }_n+{\Gamma }_f)}$. 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.