Fission-Excitation Functions in Interactions of ${\mathrm{C}}^{12}$, ${\mathrm{O}}^{16}$, and ${\mathrm{Ne}}^{22}$ With Various Targets

The fission cross sections ${\sigma }_f$ in the bombardment of Cs, Pr, Tb, Ho, ${\mathrm{Er}}^{170}$, Tm, ${\mathrm{Yb}}^{174}$, Lu, ${\mathrm{W}}^{182}$, Au, and Bi with ${\mathrm{O}}^{16}$, Tm with ${\mathrm{C}}^{12}$, and Tb with ${\mathrm{Ne}}^{22}$ have been measured as a function of projectile energy. The technique consists of counting coincident fission-fragment pairs with two Au surface-barrier Si detectors. The results are given in units of the total interaction cross section ${\sigma }_R$, and as a function of the excitation energy $E$ of the compound nucleus. It is demonstrated that for a constant value of $E$ for a compound nucleus $\frac{{\sigma }_f}{{\sigma }_R}$ is a function of the mass of the ion used and thus of the angular momentum of the nucleus. Fission for the systems investigated takes place only for nuclei formed in a complete fusion of the ion and the target nuclei. The cross section of ${\sigma }_{\mathrm{CF}}$ for this process is shown to be nearly independent of $E$ and the target used. We find $\frac{{\sigma }_{\mathrm{CF}}}{{\sigma }_R}$ to be 0.70 and 0.45 for ${\mathrm{O}}^{16}$ and ${\mathrm{Ne}}^{22}$, respectively. From the ratio $\frac{{\sigma }_f}{{\sigma }_{\mathrm{CF}}}$, experimental $\frac{{\Gamma }_f}{{\Gamma }_n}$ values are obtained and compared to theoretical ones. The following values in MeV with a standard deviation of 2 MeV for the experimental fission threshold for a nonrotating nucleus are obtained: 34.9, 26.5, 25.1, 24.6, 24.2, 20.4, 19.8, 18.2, and 17.0 for the compound nuclei ${\mathrm{Eu}}^{149}$, ${\mathrm{Ho}}^{157}$, ${\mathrm{Ta}}^{175}$, ${\mathrm{Re}}^{181}$, ${\mathrm{Os}}^{186}$, ${\mathrm{Ir}}^{185}$, ${\mathrm{Pt}}^{190}$, ${\mathrm{Au}}^{191}$, and ${\mathrm{Po}}^{198}$, respectively. These values, when corrected for shell effects, fit well the formula for a nonrotating charged liquid drop: