Learning about the nuclear symmetry energy through the lens of isospin transport

Examining nucleon transport between nuclei in intermediate energy heavy-ion collisions is an effective means to assess the density dependence of the nuclear symmetry energy. Overlap of the Fermi tails of the two nuclei as they collide provides a density gradient that drives nucleon transport. In addition, nucleon transport is driven by gradients in N/Z. Disentangling these two contributions provides a measure of the symmetry energy and its density dependence and requires a comparison of N/Z symmetric and asymmetric systems. To address this question we have examined semi-peripheral collisions of $^{\mathrm{64}}$Zn ions with $^{\mathrm{64}}$Zn, $^{\mathrm{209}}$Bi, and $^{\mathrm{27}}$Al targets at E$_{\mathrm{lab}}$ $=$ 45 MeV/A. The projectile-like fragment emerging from these collisions frequently undergoes binary decay in a dynamical fission process. By using the rotation of the projectile-like fragment as a clock, it is deduced that N/Z equilibration persists up to 1200 fm/c. As prior measurements were restricted to timescales of less than 100 fm/c, this approach represents a dramatic improvement in the sensitivity to long timescales. This work is supported by the U.S. DOE under Grant No. DEFG02-88ER-40404.