Isomeric-to-prompt fission ratios for the uranium fission isomersUm236andUm238

Fission isomers in ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ have been populated in ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}(d, p){}_{}{}^{236}{}_{}{}^{}\mathrm{U}_{}^m{}_{}{}^{}$, ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}(d, pn){}_{}{}^{236}{}_{}{}^{}\mathrm{U}_{}^m{}_{}{}^{}$, and the ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(d, pn){}_{}{}^{238}{}_{}{}^{}\mathrm{U}_{}^m{}_{}{}^{}$ reactions. Time spectra of fission fragments with respect to the cyclotron beam pulses have been measured. From these measurements the isomeric-to-prompt fission ratios are derived for deuteron energies ranging from 17 to 25 MeV. The isomeric-to-prompt ratios show a distinct maximum around 20 MeV for the ($d, \mathrm{pn}$) reactions, whereas a monotonic decrease is observed for the ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}(d, p){}_{}{}^{236}{}_{}{}^{}\mathrm{U}_{}^m{}_{}{}^{}$ reaction.