Recent measurements of the ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ neutron capture cross section show a considerable amount of fluctuation between 5 and 100 keV. Statistical tests performed on the measured cross section suggest that the fluctuations are not compatible with the statistical model of nuclear reactions and imply the existence of intermediate structure, which is interpreted in terms of doorway states. The validity of the statistical tests used here is confirmed by numerical experiments with simulated cross sections generated by the Monte Carlo method, in accordance with the statistical method. The behavior of the capture cross section is compared to those of the subthreshold fission cross section and of the inelastic scattering cross section. The threshold anomaly at the opening of the inelastic channel at 45 keV is seen in the form of a rounded step.

NUCLEAR REACTIONS Neutron capture ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$($n$,$\gamma$). $E=5\mathrm{} \mathrm{keV}-0.1\mathrm{} \mathrm{MeV}$ measured $\sigma \mathrm{}\left(E\right)\mathrm{}$; statistical tests for intermediate structure; doorway states.

• Received 20 October 1978

DOI:https://doi.org/10.1103/PhysRevC.20.528