Fission product yields for 44 masses were determined for the fission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with essentially monoenergetic neutrons of 1.5, 2.0, 3.9, 5.5, 6.9, and 7.7 MeV. Fission product activities were measured by Ge(Li) $\gamma$-ray spectrometry of irradiated ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ foils and by chemical separation of the fission product elements followed by $\beta$ counting and/or $\gamma$-ray spectrometry. The mass-yield data for ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}\mathrm{}(n,f)\mathrm{}$ show clearly the sensitive exponential increase of fission yields in the near-symmetric mass region (valley) with increasing incident neutron energy $E_n$ (peak-to-valley ratio decreasing from ∼600 to ∼40) over the range $E_n=1.5\mathrm{} \mathrm{to} 7.7$ MeV with little change in yields in other regions of the mass distribution. The valley yields plotted semilogarithmically as a function of $E_n$ reveal an abrupt change in slope at the onset of second-chance fission just above the neutron binding energy (at ∼6 MeV) where the excitation energy is lowered by competition with neutron evaporation prior to fission. Systematics are developed to permit synthesis of the mass distribution for any neutron energy in the range of 1.5 to 18 MeV.

[NUCLEAR REACTIONS, FISSION ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}\mathrm{}(n,f)\mathrm{} E_n=1.5,2.0,3.9,5.5,6.9,\mathrm{and} 7.7$ MeV; measured mass yields.]

• Received 19 September 1977

DOI:https://doi.org/10.1103/PhysRevC.17.163