Photofission yield measurements on ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and${}_{}{}^{235,236,238}{}_{}{}^{}\mathrm{U}$, showing the "shelf effect," have been analyzed in terms of a double-humped fission barrier. From the characteristic of the shelf it was possible to evaluate the excitation energy $E_{\mathrm{II}}$ of the shape isomer. In the framework of the double-humped barrier, with the competition between $\gamma$ decay to the shape isomer and tunneling through the outer barrier of a compound state in the second well, it was possible to deduce fission branching ratios in agreement with those known in the literature. In particular for ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ a three-humped fission barrier is proposed, the second deep minimum causing the shelf effect and the third one the narrow resonances detected in this isotope.

NUCLEAR REACTIONS, FISSION ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, ${}_{}{}^{235,236,238}{}_{}{}^{}\mathrm{U}$ photofission yields measurements analyzed in the frame of a double-humped fission barrier model. Excitation energies and fission branching ratios for shape isomers deduced. A three-humped fission barrier proposed for ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$.

• Received 15 November 1978

DOI:https://doi.org/10.1103/PhysRevC.20.1059