Isoscaling of heavy projectile residues and N/Z equilibration in peripheral heavy-ion collisions below the Fermi energy

The isoscaling of heavy projectile residues from peripheral heavy-ion reactions at 15-25 MeV/nucleon is employed to obtain information on the process of N/Z equilibration. Recent mass spectrometric data of projectile residues from the reactions of Kr-86 (15 MeV/nucleon) with Ni-64,Ni-58 and Sn-124,Sn-112 were first analyzed. The isotopically resolved yield distributions of the fragments in the range Z = 26-39 were employed for the isoscaling analysis. The yield ratios R-21(N, Z) of the fragments from each pair of systems exhibit isoscaling (i. e., an exponential dependence on the fragment neutron number N for each atomic number Z) with the isoscaling parameter a increasing with decreasing (or increasing) Z away from the projectile. This variation is related to the evolution toward N/Z equilibration with increasing energy dissipation estimated from the residue velocities. In parallel to the new heavy-residue isoscaling data of 86Kr at 15 MeV/nucleon, our previous data at 25 MeV/nucleon for the reactions Kr-86+ Sn-124,Sn-112 and Ni-64+ Ni-64,Ni-58, Ni-64+ Sn-124,Sn-112, as well as our data at 15 MeV/nucleon of the lighter system 40Ar+ 64,58Ni, were analyzed in a similar way. Calculations with the stochastic nucleon-exchange model DIT (deep inelastic transfer) and the microscopic many-body model CoMD (constrained molecular dynamics) provided an overall fair description of data and valuable guidance for their interpretation. Interestingly, the data of the Kr-86+ Ni, Sn reactions at 15 MeV/nucleon show a retardation of the process of N/Z equilibration which, as suggested by the CoMD calculations, is indicative of the collective character of the process. This retardation is not present in the investigated systems at 25 MeV/nucleon (and the light 40Ar+ Ni systems at 15 MeV/nucleon), whose behavior is found to be consistent with stochastic nucleon exchange.