Onset of nonequilibrium phenomena in fusionlike processes forO16-induced reactions

We have measured velocity spectra of evaporation residues from the reactions ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + Al, Ca, and Ni at bombarding energies of 8.8, 13.6, and 19.6 MeV/nucleon. Comparison with statistical model predictions shows clear evidence for the onset of incomplete momentum transfer at about 5 MeV/nucleon above the interaction barrier. To first order, the results are similar for all targets, suggesting that the missing momentum is mainly associated with the projectile. The fraction of transferred linear momentum appears to decrease linearly with increasing relative velocity of the colliding nuclei at the barrier.