Binary Fission Studies of Helium-Ion-Induced Fission of Bi209, Ra226, and U238

Abstract

The kinetic energies of coincident fission fragments were measured for helium-ion-induced fission of ${\mathrm{Bi}}^{209}$, ${\mathrm{Ra}}^{226}$, and ${\mathrm{U}}^{238}$ with gold-surface barrier detectors and an Argonne three-parameter analyzer. The data obtained at each bombarding energy were analyzed to give total kinetic-energy distributions as a function of heavy fragment mass $M_H$. The mass yield distributions for ${\mathrm{Bi}}^{209}$, ${\mathrm{U}}^{238}$, and ${\mathrm{Ra}}^{226}$ are single, double, and triple humped, respectively. The total kinetic-energy release for ${\mathrm{Bi}}^{209}$ fission decreases smoothly with increasing values of $M_H$ and its variance remains constant. The total kinetic energy as a function of $M_H$ for all the bombardments of ${\mathrm{Ra}}^{226}$ and ${\mathrm{U}}^{238}$ show structure with a maximum value at $M_H=135\mathrm{}$, while each bombardment gives a maximum in the variance of the total kinetic energy at $M_H=131\mathrm{}$. This structure in the kinetic energy for ${\mathrm{Ra}}^{226}$ and ${\mathrm{U}}^{238}$ is interpreted in terms of shell structure in the heavy fragment. The larger kinetic energy observed for symmetric fission of heavy elements with energetic particles over that observed for thermal neutron fission is assumed to be due to a smaller effective separation of charge centers at the scission configuration. This effect may possibly result from a temperature-dependent viscosity and tensile strength of the nuclear fluid and leads to the interesting speculation that information on these parameters may be inferred from nuclear fission data. The full width at half-maximum height in the total kinetic-energy distribution for 42-MeV helium-ion-induced fission of ${\mathrm{Bi}}^{209}$ is 16±1 MeV (corrected for neutron emission and experimental dispersion) in excellent agreement with a theoretical calculation of Swiatecki and Nix.