Isoscaling of heavy projectile residues and N/Z equilibration in peripheral heavy-ion collisions below the Fermi energy
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2014 American Physical Society. 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 Kr86 (15 MeV/nucleon) with Ni64,58 and Sn124,112 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 R21(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 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 Kr86 at 15 MeV/nucleon, our previous data at 25 MeV/nucleon for the reactions Kr86+Sn124,112 and Ni64+Ni64,58, Ni64+Sn124,112, as well as our data at 15 MeV/nucleon of the lighter system Ar40+Ni64,58, 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 Kr86+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 Ar40+Ni systems at 15 MeV/nucleon), whose behavior is found to be consistent with stochastic nucleon exchange.