Measurements of the ²³⁸U(n, γ) Cross Section at Thermal and Fast Neutron Energies

The capture cross section of ²³⁸U was measured using the activation technique and ²³⁵U(n, f) and ¹⁹⁷Au(n, γ) as reference cross sections. Capture events were measured by detection of two prominent gamma-ray transitions in the decay of the ²³⁹U daughter nuclide, ²³⁹Np, employing a high-resolution Ge(Li) detector. The system was calibrated with the absolutely calibrated alpha-particle emitter, ²⁴³Am, which decays to ²³⁹Np. Cross-section measurements were carried out at thermal neutron energy and in the neutron energy range from 30 keV to 3 MeV. Emphasis in the higher keV range was on absolute values between 0.14 keV and 1 MeV where the ²³⁸U(n, γ) cross section and its ratio to ²³⁵U(n, f) are not very sensitive to energy scale uncertainties, and the ²³⁸U(n, f) cross section is small. Background from fission products was found to restrict the accuracy of the measured data at energies 1.5 MeV.