  In heavy nuclei the spatial distribution of protons and neutrons is
different. At CERN SPS energies production of $\pi^+$ and $\pi^-$ 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. It may open a possibility for determination of neutron density
distribution in nuclei.
