Fission product yields for 37 masses were determined for the fission of "U with essentially monoenergetic neutrons of 0.17, 0.55, 1.0, 2.0, 4.0, 5.5, 6.3, 7.1, and 8.1 MeV. Fission product activities were measured by Ge(Li) y-ray spectrometry of irradiated 'U targets and by chemical separation of the fission product elements followed by P counting and/or y-ray spectrometry. The mass-yield data show a sensitive increase of fission yields in the near symmetric mass region (valley) with increasing incident neutron energy E" (peak-to-valley ratio decreasing from 590 to 13) over the range of 0.17 to 8.1 MeV with only small changes in yields in other regions of the mass distribution. Curves of valley yields as a function of E"display a fiat step in the region of second-chance fission (above -6 MeV) where the excitation energy is lowered by competition with neutron evaporation prior to fission. Comparison is made with monoenergetic-neutron-induced fission of 'U.
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