Using symmetric 112Sn+112Sn, 124Sn+124Sn collisions as references, we probe isospin diffusion in peripheral asymmetric 112Sn+124Sn, 124Sn+112Sn systems at an incident energy of E/A=50 MeV. Isoscaling analyses imply that the quasiprojectile and quasitarget in these collisions do not achieve isospin equilibrium, permitting an assessment of isospin transport rates. We find that comparisons between isospin sensitive experimental and theoretical observables, using suitably chosen scaled ratios, permit investigation of the density dependence of the asymmetry term of the nuclear equation of state.

Isotope, isotone and isobar yield ratios are utilized to obtain an estimate of the isotopic composition of the gas phase, i.e., the relative abundance of free neutrons and protons at breakup. Within the context of equilibrium calculations, these analyses indicate that the gas phase is enriched in neutrons relative to the liquid phase represented by bound nuclei.

Evaporation residue and fission cross sections of radioactive 132 Sn on 64 Ni were measured near the Coulomb barrier. A large subbarrier fusion enhancement was observed. Coupled-channel calculations, including inelastic excitation of the projectile and target, and neutron transfer are in good agreement with the measured fusion excitation function. When the change in nuclear size and shift in barrier height are accounted for, there is no extra fusion enhancement in 132 Sn + 64 Ni with respect to stable Sn + 64 Ni. A systematic comparison of evaporation residue cross sections for the fusion of even 112−124 Sn and 132 Sn with 64 Ni is presented. DOI: 10.1103/PhysRevC.75.054607 PACS number(s): 25.60.−t, 25.60.Pj 0556-2813/2007/75(5)/054607(9) 054607-1

Collisions of112 Sn and 124 Sn nuclei, which differ in their isospin asymmetry, provide information about the rate of isospin diffusion and equilibration. While several different probes can provide accurate diffusion measurements, the ratios of the mirror nuclei may be the simplest and most promising one. Ratios of the mass seven mirror nuclei yields are analyzed to show the rapidity, transverse momentum and impact parameter dependence of isospin diffusion. [4][5][6][7] for its determination. Recently, constraints on the density dependence of the symmetry energy were obtained from measurements of isospin diffusion in peripheral nuclear collisions [6,8]. In this paper, we identify a set of experimental observables, specifically observables constructed with yield ratios of mirror nuclei, that provide consistent measures of the isospin diffusion and extend those experimental investigations to a wider range of rapidity, transverse momentum and impact parameter.In a heavy ion collision involving a projectile and a target with different proton fractions, Z/A, the symmetry energy tends to propel the system towards isospin equilibrium so that the difference between neutron and proton densities is minimized [7].The isospin asymmetry A Z N − = δ of a projectile-like residue produced in a peripheral collision reflects the exchange of nucleons with the target; significant diffusion rates should lead to larger isospin asymmetries for collisions with neutron-rich targets and smaller isospin asymmetries for collisions with proton-rich targets [6].To isolate the isospin diffusion effects from similar effects caused by preequilibrium emission, Coulomb or sequential decays, relative comparisons involving different targets are important. In recent studies, isospin diffusion has been measured by "comparing" A+B collisions of a neutron-rich (A) nucleus and a proton-rich (B) nucleus to symmetric collisions involving two neutron-rich nuclei (A+A) and two proton-rich (B+B) nuclei under the same experimental conditions [6]. Non-isospin diffusion effects such as preequilibrium emission from a neutron-rich (A) projectile should be approximately the same for asymmetric A+B collisions as for symmetric A+A collisions. 2Similarly, non-isospin diffusion effects from a proton-rich (B) projectile in B+A collisions and B+B collisions should be the same.The degree of isospin equilibration can be quantified by rescaling the isospin asymmetry δ of a projectile-like residue from a specific collision according to the isospin transport ratio R i (δ) [6,9] given by ( )In the absence of isospin diffusion, the asymmetry B A+ δ of a residue of a neutron-rich projectile following a collision with a proton-rich target has the limiting values 1 ) (. On the other hand, if isospin equilibrum is achieved for roughly equal sized projectile and. By focusing on the differences in isospin observables between mixed and symmetric systems, R i (δ) largely removes the sensitivity to preequilibrium emission and enhances the sensitivity to isospin diffusion.Id...

We have discovered an error in our data analysis that affects the result presented in our recent Letter. The evaporation residue cross sections were calculated using the residue yield and the integrated beam. Because elements of the data array used to store the integrated beam did not have sufficient range (maximum 2 16 ), an overflow caused the integrated beam to be counted incorrectly. This was discovered by repeating some of the measurements and checked by reanalyzing the previous data with an appropriately sized data array, and using the originally sized data array but breaking up the analysis into smaller subsets. The correct cross section is presented here in Fig. 1 which replaces Fig. 2 of the original Letter. The size of the correction increases as the beam energy decreases because measurements at lower energies take longer times. At the lowest energy, the corrected cross section is a factor of 4 less than the previously published value. Since a thick target was used, the effective reaction energy was deduced by taking a cross-section-weighted average over the range of the energy loss in the target. The corrected excitation function has a steeper slope at lower energies; therefore, the calculated effective reaction energy is shifted to higher values. Fusion is still enhanced in 132 Sn on 64 Ni at sub-barrier energies with respect to a one-dimensional barrier penetration model prediction. The enhancement of fusion relative to lighter Sn isotopes is no larger than would be expected due to the larger nuclear radius of 132 Sn and transfer does not appear to play a major role in the sub-barrier fusion for this system.[1] W. S. Freeman et al., Phys. Rev. Lett. 50, 1563 (1983.[2] R. Bass, Nucl. Phys. A231, 45 (1974). FIG. 1 (color online). Fusion-evaporation excitation functions of 132 Sn 64 Ni and 64 Ni on even 112-124 Sn [1]. The reaction energy is scaled by the fusion barrier predicted by the Bass model [2] and the evaporation residue (ER) cross section is scaled by the size of the reactants using R 1:2A 1=3 p A 1=3 t , where A p (A t ) is the mass of the projectile (target). The filled circles are corrected data and the open circle is our measurement using a 124 Sn beam.