Variation inU235Mass Yields at Neutron Energies Below 0.5 eV

Variations in the shape of the fission-product mass-yield distribution curve when ${\mathrm{U}}^{235}$ is irradiated with neutrons of various energies up to 0.5 eV were studied by radiochemical measurements of peak-to-valley ratios. A >30% increase in the relative amount of symmetric fission was found at the peak of the 0.29-eV resonance compared to the amount of symmetric fission produced with thermal neutrons. A decrease in the amount of symmetric fission was observed at increasing neutron energies where symmetric fission was 18% less common with epicadmium pile neutrons than with those at thermal energy. At neutron energies below 0.01 eV a sudden increase in the relative amount of symmetric fission was also found, even though there is no evidence from cross-section data for a resonance in this energy region. The results of this study are compared with those of other investigations at higher neutron energies. A discussion is presented regarding implications of mass-yield variations in the theory of fission through saddle-point states.