Photofission cross sections for ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, and ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ have been measured in the energy range from 3.25 to 5.75 MeV and for ${}_{}{}^{234}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ at 3.5 MeV. The cross sections change by over seven orders of magnitude for this energy range. Cross section shapes are significantly different for different isotopes indicating a strong sensitivity to fission barrier parameters.

NUCLEAR REACTIONS, FISSION Barrier shape; fission; nuclear reactions; photofission bremsstrahlung; threshold; tract counting.

• Received 25 October 1977

DOI:https://doi.org/10.1103/PhysRevC.17.1086