Symmetry of Neutron-InducedU235Fission at Individual Resonances

Neutrons in the resonance energy region from a nuclear explosion were resolved by time-of-flight and used to induce fissions in ${\mathrm{U}}^{235}$ attached to a revolving wheel. The symmetry of fission at individual resonances from approximately 10 to 60 ev was examined by radiochemical means. As measured by the ratio ${\mathrm{Ag}}^{111}$/${\mathrm{Mo}}^{99}$, the probability of symmetric fission decreased at some resonances by a maximum of 10% compared to thermal fission of ${\mathrm{U}}^{235}$ and at other resonances increased by a maximum of 40%. With varying degrees of assurance, nine resonances are identified with an increase in symmetry; five more regions of increased symmetry are associated with resonances or a background effect. Twenty resonances are identified with a decrease in symmetry. In a sample containing 500 levels in the resonance region, there was no level with a ${\mathrm{Ag}}^{111}$ yield even one-thirtieth as great as ${\mathrm{Mo}}^{99}$. It is thus very improbable that there are any neutron resonances in ${\mathrm{U}}^{235}$ that lead to predominantly symmetric fission.