  The theoretical nuclear physics predictions and experimental results on
antiprotonic atoms lead to different proton and neutron spatial distributions.
At CERN SPS energies production of positive and negative pions differs for
$pp$, $pn$, $np$ and $nn$ scattering. These two facts lead to an impact
parameter dependence of the $\pi^+$ to $\pi^-$ ratio in $^{208}Pb + ^{208}Pb$
collisions. A recent experiment at CERN seems to confirm qualitatively these
predictions. The results for two models considered are almost identical. It may
open a new possibility for determination of neutron density distribution in
nuclei.
