Total Cross Sections for Fission ofU238Induced byHe4and Heavy Ions

The total fission cross sections have been measured for bombardment of ${\mathrm{U}}^{238}$ with ${\mathrm{He}}^4$, ${\mathrm{B}}^{11}$, ${\mathrm{C}}^{12}$, ${\mathrm{N}}^{14}$, ${\mathrm{O}}^{16}$, and ${\mathrm{Ne}}^{20}$ ions at energies up to 10.4 MeV/nucleon. Because of the high fissionability of these systems, it is assumed that the fission cross section is equal to the total reaction cross section for heavy-ion reactions. The data have been compared with the theoretical cross-section calculations of Thomas, assuming (1) a square-well nuclear potential, and (2) a parabolic approximation to the real part of the optical potential. At energies well above the Coulomb barrier, the data are well represented using a square-well potential and $r_0=1.50\mathrm{}$F. Near the barrier, however, the agreement is poor. With the parabolic approximation, the entire excitation function can be generally reproduced except in the case of ${\mathrm{Ne}}^{20}$. For the ${\mathrm{He}}^4$ data, these calculations used a well depth $V_0=-67\mathrm{}$ MeV, a nuclear radius $r_0=1.17\mathrm{}$F, and a diffuseness parameter $d=0.574\mathrm{}$F. These values for heavy ions were $V_0=-70\mathrm{}$ MeV, $r_0=1.23\mathrm{} \mathrm{to} 1.26$F, and $d=0.50\mathrm{} \mathrm{to} 0.44$F, $r_0$ increasing and $d$ decreasing as a function of increasing heavy-ion mass.