Inelastic scattering of fast neutrons from 238U

The cross-sections for inelastic scattering of fast neutrons from 238U are of considerable importance in reactor calculations. In spite of this fact, relatively few detailed measurements of these quantities have been reported. The need for another independent measurement of the excitation function for inelastic scattering from threshold up to several MeV has been pointed out on several occasions. The present experiment was undertaken in an attempt to check the accuracy of existing data in the energy region below about 1.2 MeV. Neutrons with an energy spread of less than 10 keV were obtained from a pulsed 3 MV Van-de-Graaff accelerator and the 7Li (p, n)7Be reaction. Time-of-flight techniques were employed and care was taken to ensure sufficient resolution of the neutron groups corresponding to the excitation of different energy levels in 238U. Results were obtained for the levels at approximately 45, 150, 680 and 730 keV. All cross- sections were determined relative to the known cross-section for, elastic scattering from carbon. Corrections were applied for flux attenuation and multiple scattering. The excitation curve for the first excited state in 238U, measured in steps of 10 keV (or less) in incident energy, rises fairly smoothly with energy and only slight indications of structure were observed. The absolute values of the 90o differential inelastic-scattering cross-section for all the energy levels agree fairly well with the detailed results published by the Harwell group in 1966. In view of the completely different methods of cross-section normalization employed in these two experiments, this agreement is encouraging. (author)