Properties of the reaction 238U(n, f) at the vibrational resonances

Recent fission cross-section calculations for the reaction U-238(n, f), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by Delta A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron enenmy E-n = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard I (S1) and standard 2 (S2) as well as the Symmetric super-long (SL) mode. In order to verily the model predictions, a dedicated experiment on U-238 has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from E-n = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at E-n = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around E-n = 1.2 and 1.6 MeV was observed at E-n = 0.9 MeV as well. The predicted large changes in fission fagment mass yield and total kinetic energy Could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at E-n = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode. (C) 2008 Elsevier B.V. All rights reserved.