Equilibration and equilibration rates have been measured by colliding Sn nuclei with different isospin asymmetries at beam energies of $E/A=35$ MeV. Using the yields of mirror nuclei of ${}^7\mathrm{Li}$ and ${}^7\mathrm{Be}$, we have studied the diffusion of isospin asymmetry by combining data from asymmetric ${}^{112}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ and ${}^{124}\mathrm{Sn}+{}^{112}\mathrm{Sn}$ collisions with those from symmetric ${}^{112}\mathrm{Sn}+{}^{112}\mathrm{Sn}$ and ${}^{124}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ collisions. We use these measurements to probe isospin equilibration in central collisions where nucleon-nucleon collisions are strongly blocked by the Pauli exclusion principle. The results are consistent with transport theoretical calculations that predict a degree of transparency in these collisions, but inconsistent with the emission of intermediate mass fragments by a single chemically equilibrated source. Comparisons with quantum molecular dynamics calculations are consistent with results obtained at higher incident energies that provide constraints on the density dependence of the symmetry energy.

• Received 7 September 2010

DOI:https://doi.org/10.1103/PhysRevC.82.051603