  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 7Li and 7Be, we have studied the diffusion of
isospin asymmetry by combining data from asymmetric 112Sn+124Sn and 124Sn+112Sn
collisions with that from symmetric 112Sn+112Sn and 124Sn+124Sn collisions. We
use these measurements to probe isospin equilibration in central collisions
where nucleon-nucleon collisions are strongly blocked by the Pauli exclusion
principal. 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 ImQMD calculations are consistent with
results obtained at higher incident energies that provide constraints on the
density dependence of the symmetry energy.
