Correlations between projectilelike and targetlike fragments have been measured in the reaction 27 Al + 44-MeV/nucleon 40 Ar. These correlations can be consistently interpreted in the framework of an abrasion model including dissipation. However, they can also be described by a binary process in which, before decaying sequentially by particle emission, projectile and target share an approximately equal amount of excitation energy with no significant mass transfer.PACS numbers: 25.70.Np For projectiles of intermediate energies (20 to 100 MeV/nucleon), peripheral collisions between heavy ions have recently been subjected to extensive studies, 1 " 4 motivated by a possible rapid change 5 in the reaction mechanism as the projectile velocity becomes comparable to the velocity of the nucleons inside the nucleus. Such a transition is suggested by a rapid broadening of the projectilelike-fragment (PLF) momentum distributions 6 between 20-and 40-MeV/nucleon projectile energy. This increase in momentum width is expected in a transition from transfer to abrasion, due to the opening of the phase space made available to the nucleons removed from the projectile. 7,8 Very schematically, for projectile energies smaller than --20 MeV/nucleon, most of these fragments are considered as the remnants of an incomplete fusion process in which only part of the projectile fuses with the target, 9 wheres for energies greater than 200 MeV/nucleon, they are thought to be produced by a fast removal (abrasion) of the nucleons in the region of overlap between projectile and target. 10 Thus, it would be interesting to determine over which energy range the transition between those two processes occurs. However, the other PLF properties (angular distributions, isotopic ratios, energy damping) evolve smoothly from the low-energy regime to relativistic energies, showing hardly any evidence of a change in the reaction process. Thus, PLF properties alone do not appear as a sensitive probe of the reaction mechanism. A better signature could be the amount of linear momentum imparted to the target since in the hypothesis of massive transfer, the nucleons removed from the projectile are captured by the target to which they communicate their linear momentum, whereas in the case of abrasion, only a small fraction of this momentum is imparted to the target through friction forces. Thus, the onset of abrasion should manifest itself by a sharp drop in the linear momentum imparted to the targetlike fragments (TLF's).To better determine the underlying reaction mechanism in the expected transition energy region, we have measured the mass, angular, and velocity correlations between PLF's and TLF's in the reactions induced by 1760-MeV 40 Ar on 27 A1, for which there already exist extensive inclusive data on PLF's. {x The experiment was performed using a 44-MeV/ nucleon 40 Ar beam at the Grand Accelerateur National d'lons Lourds facility. In order to reduce energy and angular straggling of the TLF's the self-supported 27 A1 target was only 100 jug/cm 2 thick. ...
