Anomalies in the Charge Yields of Fission Fragments from the 238U(n, f) Reaction

Fast-neutron-induced fission of U-238 at an energy just above the fission threshold is studied with a novel technique which involves the coupling of a high-efficiency.-ray spectrometer (MINIBALL) to an inversekinematics neutron source (LICORNE) to extract charge yields of fission fragments via gamma-gamma coincidence spectroscopy. Experimental data and fission models are compared and found to be in reasonable agreement for many nuclei; however, significant discrepancies of up to 600% are observed, particularly for isotopes of Sn and Mo. This indicates that these models significantly overestimate the standard 1 fission mode and suggests that spherical shell effects in the nascent fission fragments are less important for low-energy fastneutron-induced fission than for thermal neutron-induced fission. This has consequences for understanding and modeling the fission process, for experimental nuclear structure studies of the most neutron-rich nuclei, for future energy applications (e.g., Generation IV reactors which use fast-neutron spectra), and for the reactor antineutrino anomaly.