U238neutron-induced fission cross section for incident neutron energies between 5 eV and 3.5 MeV

Abstract

A measurement of the $^{238}\mathrm{U}$ neutron-induced fission cross section has been performed in the neutron energy range between 5 eV and 3.5 MeV. The favorable signal-to-background ratio and high resolution of this experiment resulted in the identification of 85 subthreshold fission resonances or clusters of resonances in the neutron energy region between 5 eV and 200 keV. The fission data below 100 keV are characteristic of a weak coupling situation between Class I and Class II levels. The structure of the fission levels at the 720- and 1210-eV fission clusters is discussed. There is an apparent enhancement of the fission cross section at the opening of the ${2}^{+}$ neutron inelastic channel in $^{238}\mathrm{U}$ at 45 keV. An enhancement of the subthreshold fission cross section between 100 and 200 keV has been tentatively interpreted in terms of the presence of a Class II, partially damped vibrational level. There is a marked structure in the fission cross section above 200 keV up to and including the plateau between 2 and 3.5 MeV.NUCLEAR REACTIONS, FISSION $^{238}\mathrm{U}$($n$,$f$). $E=5 \mathrm{eV}\ensuremath{-}3.5 \mathrm{MeV}$; measured $\ensuremath{\sigma}(E)$; deduced parameters second well fission barrier.