Isoscaling of heavy projectile residues andN/Zequilibration in peripheral heavy-ion collisions below the Fermi energy

Abstract

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 $^{86}\mathrm{Kr}$ (15 MeV/nucleon) with $^{64,58}\mathrm{Ni}$ and $^{124,112}\mathrm{Sn}$ 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 $\ensuremath{\alpha}$ 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 $^{86}\mathrm{Kr}$ at 15 MeV/nucleon, our previous data at 25 MeV/nucleon for the reactions $^{86}\mathrm{Kr}+^{124,112}\mathrm{Sn}$ and $^{64}\mathrm{Ni}+^{64,58}\mathrm{Ni}$, $^{64}\mathrm{Ni}+^{124,112}\mathrm{Sn}$, as well as our data at 15 MeV/nucleon of the lighter system $^{40}\mathrm{Ar}+^{64,58}\mathrm{Ni}$, 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 $^{86}\mathrm{Kr}+\mathrm{Ni},\mathrm{Sn}$ 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 $^{40}\mathrm{Ar}+\mathrm{Ni}$ systems at 15 MeV/nucleon), whose behavior is found to be consistent with stochastic nucleon exchange.