Low-Yield Products from Fission of Th232 , U235 , and U238 with 14.8-MeV Neutrons

We have measured the fission yields of a number of products from 14.8-MeV neutron fission of ${\mathrm{Th}}^{232}$, ${\mathrm{U}}^{235}$, and ${\mathrm{U}}^{238}$. The fission products chosen are all on the wings of the mass-yield curves and are formed in very low yield. They extend from ${\mathrm{Ni}}^{66}$ to ${\mathrm{Zn}}^{72}$ and from ${\mathrm{Sm}}^{153}$ to ${\mathrm{Er}}^{172}$. The amount formed of each product was determined by absolute $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ counting techniques. The number of fissions in each target was calculated from the target mass, the fission cross section, and the neutron flux. The neutron flux was measured by means of the ${\mathrm{Y}}^{89}(n,2n){\mathrm{Y}}^{88}$ reaction with ${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$ monitor foils. The results show that, within experimental uncertainty, the wings of the mass-yield curves are consistent with Gaussian functions. These Gaussian curves allow interpolation and prediction of fission yields of unmeasured products. The widths of the massyield curves for ${\mathrm{U}}^{235}$ and ${\mathrm{U}}^{238}$ are almost the same, while that of ${\mathrm{Th}}^{232}$ is significantly narrower. The centers of the Gaussian distributions are shifted to higher mass numbers than would be predicted from the average total neutron emission in fission. The effect of target impurities on the measured fission yields was shown to be generally small. An attempt was made to examine the effect of nuclear charge distribution on the mass yields. This effect, which would cause the observed fission yields to be less than the total mass yield, is probably significant only for the yields of masses 166 and 172. As a check on our experimental method we also remeasured the fission yields of three products near the peaks of the mass-yield curves. Our results are consistent with those reported before.