Nucleon exchange in the absence of strong driving forces: The reaction ${}_{}{}^{238}{}_{}{}^{}\mathrm{Ca}$ at $E_{\mathrm{lab}=425\mathrm{}}$ MeV

Mass number, atomic number, energy, and angular distributions have been measured for the projectile-like fragments from the damped reaction ${}_{}{}^{238}{}_{}{}^{}$U${+}^{48}$Ca at $E_{\mathrm{lab}=425\mathrm{}}$ MeV. Correlations in the measured two-dimensional probability distribution P(A,Z) are discussed along with the general dissipative features of the damped reaction mechanism. The first and second moments of the observed atomic and mass number distributions are compared with those of other asymmetric reaction systems. Although the ${}_{}{}^{238}{}_{}{}^{}$U${+}^{48}$Ca system is highly asymmetric, its potential energy surface in the vicinity of the injection point has a nearly zero gradient resulting in a negligible drift in the proton number. The proton drifts observed in reactions of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with five different projectiles vary systematically with the potential gradient. The mass and charge data are compared with quantitative predictions by a dynamical reaction model based on nucleon exchange. The theoretical model gives a reasonable account of the mass and charge distributions for the ${}_{}{}^{238}{}_{}{}^{}$U${+}^{48}$Ca reaction but fails systematically to predict experimental drifts when the driving forces are large.