Excitation Functions for the Helium-Ion-Induced Fission of Rhenium, Lutetium, and Thulium

The helium-ion-induced fission excitation functions of rhenium, lutetium, and thulium have been measured in the energy interval of 30 to 80 MeV using a "fission-track" technique with plastic films as detectors. By analyzing the ratio $\frac{{\Gamma }_f}{{\Gamma }_n}$, which measures competition between fission and neutron emission, fission thresholds of 22.4, 23.6, 27.5, and 28.7 MeV have been determined for ${\mathrm{Ir}}^{189}$, ${\mathrm{Ir}}^{191}$, ${\mathrm{Ta}}^{179}$ and ${\mathrm{Lu}}^{173}$, respectively. These values are obtained using a level-density parameter $a_f=\frac{A}{10}$, with the uncertainty of this choice introducing a possible error of ±2.5 MeV in the calculated barriers. The thresholds, which are of particular interest because the nuclides involved are removed from the closed-shell region, are in agreement with predictions of a recently proposed semi-empirical mass formulation based on a charged liquid drop. Correlations are given showing the relationship of $\frac{{\Gamma }_f}{{\Gamma }_n}$ with $\frac{Z^2}{A}$ and ${E_f}^{\prime }-{B_n}^{\prime }$, where ${E_f}^{\prime }$ and ${B_n}^{\prime }$ are the effective fission barrier and the neutron binding energy. The results suggest that shell effects tend to persist even at excitation energies of 30 MeV. Comparisons of the correlations with heavy-element data point out the marked discontinuity near $Z=90\mathrm{}$, and support the general two-mode fission hypothesis.