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 ΓfΓₙ and some comments are made on the effect of nuclear deformation on the Fermi gas level density parameter a. Fission thresholds for At²¹³, Po²¹⁰, Bi207,209, and Tl²⁰¹ 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 Z²A between 32 and 40.