Fission-Excitation Functions in Interactions ofC12,O16, andNe22With Various Targets

Abstract

The fission cross sections ${\ensuremath{\sigma}}_{f}$ 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 ${\ensuremath{\sigma}}_{R}$, 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 $\frac{{\ensuremath{\sigma}}_{f}}{{\ensuremath{\sigma}}_{R}}$ 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 ${\ensuremath{\sigma}}_{\mathrm{CF}}$ for this process is shown to be nearly independent of $E$ and the target used. We find $\frac{{\ensuremath{\sigma}}_{\mathrm{CF}}}{{\ensuremath{\sigma}}_{R}}$ to be 0.70 and 0.45 for ${\mathrm{O}}^{16}$ and ${\mathrm{Ne}}^{22}$, respectively. From the ratio $\frac{{\ensuremath{\sigma}}_{f}}{{\ensuremath{\sigma}}_{\mathrm{CF}}}$, experimental $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ 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: ${{E}_{f}}^{L}=6.76(0.75\ensuremath{-}x){A}^{\frac{2}{3}}$. Here, $x=\frac{(\frac{{Z}^{2}}{A})}{{(\frac{{Z}^{2}}{A})}_{\mathrm{crit}.}}$ and we obtain the value 48.0\ifmmode\pm\else\textpm\fi{}1.0 for ${(\frac{{Z}^{2}}{A})}_{\mathrm{crit}.}$. The ratio between the level-density parameters for fission and neutron evaporation $\frac{{a}_{f}}{{a}_{n}}$ is found to be 1.22\ifmmode\pm\else\textpm\fi{}0.05; this ratio is independent of nuclear type. In particular, the ratio is nearly the same inside and outside the region of the closed shell. Values for moments of inertia can not be evaluated from the analysis.