Isospin diffusion and equilibration for<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Sn</mml:mi><mml:mo>+</mml:mo><mml:mi mathvariant="normal">Sn</mml:mi></mml:mrow></mml:math>collisions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>E</mml:mi><mml:mo>/</mml:mo><mml:mi>A</mml:mi><mml:mo>=</mml:mo><mml:mn>35</mml:mn></mml:mrow></mml:math>MeV

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.