Evolution of mass and charge asymmetry in damped heavy-ion reactions

Projectile-like fragments have been studied with high A and Z resolution for the damped reactions ${}_{}{}^{238}{}_{}{}^{}$U${+}^{40}$Ca, ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, and ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$ at 8.5 MeV/nucleon. These dinuclear systems exhibit widely different driving forces. Qualitative agreement between experiment and dynamical nucleon-exchange transport theory is obtained for the first and second moments of the fragment atomic (Z) and neutron (N) number distributions. However, systematic differences exist between the experimental and theoretical average values of Z and N suggesting the need for more realistic driving forces.