Radiochemical measurements of the relative yields of the fission products ${\mathrm{Mo}}^{99}$, ${\mathrm{Ag}}^{111}$, ${\mathrm{Ag}}^{113}$, and ${\mathrm{Cd}}^{115}$ from the low-energy neutron-induced fission of ${\mathrm{U}}^{233}$ have been made at various neutron energies. The energies chosen were thermal, 1.8, 2.3, and 4.7 ev, the latter three corresponding to the peaks of previously reported resonances. It was found that the ratio of asymmetric to symmetric fission is larger at the 1.8- and 2.3-ev resonances than at thermal energies. At the 4.7-ev resonance however, this ratio is the same as at thermal energy, to within experimental uncertainties. In addition, it was found that the ratio for epi-cadmium neutrons differed from that for thermal neutrons. The results are consistent with Wheeler's prediction that the ratio of asymmetric to symmetric fission should depend upon the spin state of the fissioning nucleus.

• Received 15 October 1958

DOI:https://doi.org/10.1103/PhysRev.113.1589