The fission probability was measured for a series of actinide nuclei as a function of the excitation enery using (${}_{}{}^3{}_{}{}^{}\mathrm{He},df$) and (${}_{}{}^3{}_{}{}^{}\mathrm{He},tf$) reactions. From these data, $\frac{{\Gamma }_n}{{\Gamma }_f}$ was determined from threshold up to ∼ 12 MeV of excitation energy, and fitted by evaporation calculations which do not contain any arbitrary normalization factors. For heavier actinides, these fits are possible only if one assumes the fission process to proceed through a first saddle point which is not axially symmetric. These results and calculations also reproduce previously known empirical trends of average values of $\frac{{\Gamma }_n}{{\Gamma }_f}$, as a function of $A$ and $Z$.

NUCLEAR REACTIONS, FISSION Measured fission probability in ${}_{}{}^{230-233}{}_{}{}^{}\mathrm{Pa}$, ${}_{}{}^{231-232}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{233-239}{}_{}{}^{}\mathrm{Np}$, ${}_{}{}^{237-238}{}_{}{}^{}\mathrm{Pu}$, ${}_{}{}^{239-243}{}_{}{}^{}\mathrm{Am}$, ${}_{}{}^{241-244}{}_{}{}^{}\mathrm{Cm}$. Determined barrier heights.

• Received 26 January 1976

DOI:https://doi.org/10.1103/PhysRevC.13.2374