Effect of Increasing Excitation Energy on Nuclear Charge Distribution in Fission

The independent fractional chain yields of ${\mathrm{Y}}^{90}$, ${\mathrm{Nb}}^{96}$, and ${\mathrm{La}}^{140}$ have been measured from fission of ${\mathrm{Th}}^{232}$ with alpha particles. The excitation energy was varied from about 20 to 40 MeV. Yields are also calculated for ${\mathrm{Cs}}^{136}$ from data previously published. In addition, the independent fractional chain yields of ${\mathrm{Y}}^{92}$, ${\mathrm{Nb}}^{96}$, and ${\mathrm{La}}^{140}$ were measured from fission of ${\mathrm{U}}^{235}$ with 14.9-MeV neutrons. From these data, smoothly varying plots of independent yield versus excitation energy were constructed for ${\mathrm{Y}}^{90}$, ${\mathrm{Nb}}^{96}$, ${\mathrm{Cs}}^{136}$, and ${\mathrm{La}}^{140}$. The data from 14.9-MeV neutron fission of ${\mathrm{U}}^{235}$ are in good agreement with those from alpha-particle fission of ${\mathrm{Th}}^{232}$ at the same excitation energy. It is shown that, within the limits of certain assumptions regarding the charge dispersion curves, the shift in $Z_p$ (the most probable charge for a given mass number) with increasing excitation energy varies among the nuclides studied. In particular, it appears that $\frac{d{Z}_p}{\mathrm{dE}}$ may be significantly greater for heavy fragments than for light fragments. The data for ${\mathrm{Y}}^{90}$, ${\mathrm{Nb}}^{96}$, and ${\mathrm{La}}^{140}$ are consistent with linear plots of $Z_p$ versus $E^{*}$, except that the value for ${\mathrm{Nb}}^{96}$ for thermal-neutron fission of ${\mathrm{U}}^{235}$ is off the curve by 0.1 charge unit. The value of $\frac{d{Z}_p}{\mathrm{dE}}$ for ${\mathrm{Cs}}^{136}$ decreases with energy in this energy range, possibly because of shell effects at low excitation energies.