Symmetry of Neutron-InducedPu239Fission at Individual Resonances

Neutrons resolved in energy by time-of-flight from an underground nuclear explosion ("Petrel," 11 June 1965) have been used to produce resonance fissions in ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ in sufficient numbers to permit radiochemical measurements of the symmetry of fission at individual levels in the energy region 15 to 82 eV. About two-thirds of the levels examined are characterized by fission less symmetric than thermal-neutron fission. At these resonances the modal yield of the nearly symmetric fission product ${}_{}{}^{115}{}_{}{}^{}\mathrm{Cd}$ is approximately $\frac{1}{3}$ the thermal yield. For the remainder of the levels, fission is more symmetric; the modal yield of ${}_{}{}^{115}{}_{}{}^{}\mathrm{Cd}$ is approximately $\frac{3}{2}$ the thermal yield. There is correspondence between groupings by enhanced or decreased fission symmetry, level widths ("broad" or "narrow"), and some recently measured $J$ values. Based on relative fission symmetry, spin assignments are proposed for 22 levels. It is suggested that those levels for which the ${}_{}{}^{115}{}_{}{}^{}\mathrm{Cd}$ yield is low represent 1+ states; the higher ${}_{}{}^{115}{}_{}{}^{}\mathrm{Cd}$ yields presumably occur at levels which represent 0+ states. The 0+ levels are broad compared to the more readily recognized and categorized 1+ levels. The proposed assignments are: for the $J=1\mathrm{}$ group, resonances at 17.7, 22.3, 26.3, 41.5, 44.6, 47.7, 50.2, 52.7, 55.8, 59.4, 60.0, 75.2, and, with less confidence, 15.4 eV; for the $J=0\mathrm{}$ group, resonances at 49.8, 57.6-58, 61.1, 66.8, 81.7, and, with less confidence, 32.4 eV. In addition, from other published data, we suggest $J=1\mathrm{}$ for the 0.3-eV level and J=0 for the first negative-energy level.