Helium-Ion-Induced Fission of Bi, Pb, Tl, and Au

The fission cross sections have been measured with solid-state detectors for helium-ion-induced fission of bismuth, lead-206, thallium, and gold. The measurements were made at several helium-ion projectile energies between 30 and 43 Mev. The fission cross sections of bismuth, lead-206, thallium, and gold with 42.8-Mev helium ions are 7.3, 1.8, 0.65, and 0.28 mb, respectively, and the cross sections decrease rapidly with reduced-energy projectiles. The competition between fission and neutron emission as a function of excitation energy is compared with theoretical predictions of $\frac{{\Gamma }_f}{{\Gamma }_n}$ and some comments are made on the effect of nuclear deformation on the Fermi gas level density parameter $a$. Fission thresholds for ${\mathrm{At}}^{213}$, ${\mathrm{Po}}^{210}$, ${\mathrm{Bi}}^{207,209}$, and ${\mathrm{Tl}}^{201}$ of 15.8±2.0, 18.6±2.0, 20.6±2.0, and 19.9±2.0 Mev are derived. The saddlepoint masses of these nuclei relative to Cameron's reference mass surface lie on a smooth curve with the heavy element data, indicating that the shell structure is completely destroyed during the distortion from equilibrium to saddle-point deformation. An empirical equation for fission thresholds is deduced from the saddle-point mass surface which is thought to be valid for nuclei with $\frac{Z^2}{A}$ between 32 and 40.