Absolute cross sections for proton-induced fission of the uranium isotopes

Abstract

Absolute cross sections for proton induced fission have been measured for $^{233}\mathrm{U}$, $^{234}\mathrm{U}$, $^{235}\mathrm{U}$, $^{236}\mathrm{U}$, and $^{238}\mathrm{U}$ for proton energies ranging from 5.0 to 30.0 MeV. Angular distributions have been measured for all isotopes and the fission fragment angular anisotropy determined as a function of excitation energy. Excitation data were accumulated at 250 keV intervals with smaller steps in regions of special interest. Fission probabilities calculated from the data and optical model predictions prominently display effects due to the opening of additional fission channels as excitation energy is increased. A method of analysis is formulated and demonstrated which makes use of the extended energy range and selection of an isotopic sequence of target nuclei to unfold the contributions to the measured cross section from first, second, and third chance fission. Using this method, previously undetermined fission thresholds and $\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{f}}$ ratios are estimated for individual nuclei as functions of excitation energy.NUCLEAR REACTIONS, FISSION $^{233}\mathrm{U}(p,xnf)$, $^{234}\mathrm{U}(p,xnf)$, $^{235}\mathrm{U}(p,xnf)$, $^{236}\mathrm{U}(p,xnf)$, and $^{238}\mathrm{U}(p,xnf)$, $E=5.0\ensuremath{-}30.0$ MeV; measured ${\ensuremath{\sigma}}_{p,xnf}({E}_{p},\ensuremath{\theta})$; deduced ${\ensuremath{\sigma}}_{\mathrm{fT}}(E)$, $\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{f}}(E,A)$, fission barriers.