Mass distributions in monoenergetic-neutron-induced fission of Pu239

Fission product yields for 24 masses were determined for the fission of $^{239}\mathrm{Pu}$ with essentially monoenergetic neutrons of 0.17, 1.0, 2.0, 3.4, 4.5, 6.1, and 7.9 MeV. Fission product activities were measured by Ge(Li) $\ensuremath{\gamma}$-ray spectrometry of irradiated $^{239}\mathrm{Pu}$ targets and by chemical separation of the fission product elements followed by $\ensuremath{\beta}$ counting. Yields of near symmetric (valley) fission products increase nearly monotonically with incident neutron energy and do not exhibit a break in the slope of the yield versus neutron energy curve in the energy region where second-chance fission begins. This is in contrast to the curves for $^{232}\mathrm{Th}$, $^{235}\mathrm{U}$, and $^{238}\mathrm{U}$ neutron-induced fission.NUCLEAR REACTIONS, FISSION $^{239}\mathrm{Pu}(n, f)$, ${E}_{n}=0.17, 1.0, 2.0, 3.4, 4.5, 6.1, \mathrm{and} 7.9$ MeV; measured mass yields.