Prompt fission product yields in the U238(n,f) reaction

Background: Significant yield discrepancies (500%--600%) were reported recently between experimental results and predictions (from the gef model) and evaluations (from the JEFF-3.1.1 and ENDF/B-VII.1 libraries) for Mo and Sn fission-fragment yields in fast-neutron-induced reactions on $^{238}\mathrm{U}$ using $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence spectroscopy. The model and evaluations also predict Mo and Sn fragments that are on average $\ensuremath{\sim}1$ to 2 neutrons richer than the experimental results.Purpose: $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence spectroscopy favors detection of higher-multiplicity $\ensuremath{\gamma}$-ray cascades. An alternative approach is determining the fragment yields using single-$\ensuremath{\gamma}$-ray spectroscopy, as it was attempted here for selected cases where it was feasible. Advantages and drawbacks in both approaches need to be understood and potential systematic errors in the experimental results should be addressed using theoretical models.Methods: Fast neutrons from the LANSCE WNR facility were used to induce fission on $^{238}\mathrm{U}$. The emitted $\ensuremath{\gamma}$ rays were measured with the GEANIE spectrometer.Results: The yield of selected even-even fission fragments was determined. The selection was based on the ability to reliably determine excitation functions for the detected $\ensuremath{\gamma}$ rays.Conclusions: Our single-$\ensuremath{\gamma}$-ray results provide better agreement between experiment and predictions and evaluations.