Reaction mechanism studies with 7-35 MeV/nucleonHe4ions incident on heavy target nuclei

Reaction mechanism studies have been performed on the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ + ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$ system at energies of 7-35 MeV/nucleon and on the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ + ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ system at 12-35 MeV/nucleon. Measurements included (1) forward-angle elastic scattering, (2) angular distributions and total cross sections for fission, (3) fission-fragment angular correlations, and (4) fragment charge, mass, and energy distributions for ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$($\alpha$,$f$) at 140 MeV. Total inelastic cross sections and interaction radii were derived from the elastic scattering data. These values were found to be in good agreement with the total cross section for fission for the ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$ + ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ system, confirming the assumption that ${\sigma }_R\simeq {\sigma }_f$. Fission-fragment angular-correlation measurements were performed in order to deduce the distribution of linear momenta which characterize the residual heavy nuclei formed in these collisions. Complete linear momentum transfer probabilities (complete fusion) were also derived for both target-projectile systems as a function of energy. The data are compared with predictions of the proximity potential with one-body energy dissipation and with the intranuclear cascade code. The results of these comparisons are consistent with a picture in which complete fusion dominates the reaction mechanism below 10 MeV/nucleon, but pre-equilibrium processes assume increasing importance above this energy at the expense of fusion. In addition, evidence for a possible reduction in the interaction radius above 20 MeV/nucleon is reported.