  We use the Isospin Quantum Molecular Dynamics model (IQMD) to analyze the
centrality dependence of the isospin ratio of pions, $\pi^-/\pi^+$. We find
that the density dependence of the asymmetry potential, the Pauli blocking of
the $\Delta$-decay and the thickness of the neutron skin influence in different
ways this observable. Using the centrality dependence of this ratio at
different beam energies we can disentangle the different contributions and open
the way for their experimental determination.
