Search for low-yield products in the neutron-induced highly asymmetric fission of uranium

The results of investigations aimed at defining the nature of mass distribution in the reactor neutron-induced highly asymmetric binary fission of natural, depleted, and enriched uranium are presented. Fission yields of mass chains 66, 67, 72, 77, 161, 172, 175, 177 were determined relative to ${}_{}{}^{99}{}_{}{}^{}\mathrm{Mo}$ using radiochemical methods. Upper limits for the yields of ${}_{}{}^{183}{}_{}{}^{}\mathrm{Ta}$ and ${}_{}{}^{199}{}_{}{}^{}\mathrm{Au}$ were also set to see the trend of mass distribution beyond $A=177\mathrm{}$. As expected the yields of highly asymmetric products in the region $A<\mathrm{}70\mathrm{}$ and $A>\mathrm{}161\mathrm{}$ are sensitive to the excitation energy of the fissioning nucleus, a trend observed in symmetric fission. Computer extrapolation of the yields up to $A=180\mathrm{}$ on the heavier side and $A=60\mathrm{}$ on the lighter side indicates that both wings of the mass-yield curve cannot be represented by a single Gaussian, thereby disturbing the complementarity of the mass-yield curve in the highly asymmetric region. The relatively higher yields for mass regions around $A=67\mathrm{} \mathrm{and} 177$, together with the upper limits for $A=183\mathrm{} \mathrm{and} 199$ in the reactor neutron fission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, tend to suggest the possibility of shoulders in the low-yield wings of the mass-yield curve.