Neutron density distributions from antiprotonicPb208andBi209atoms

The x-ray cascade from antiprotonic atoms was studied for ${}^{208}\mathrm{Pb}$ and ${}^{209}\mathrm{Bi}$. Widths and shifts of the levels due to the strong interaction were determined. Using modern antiproton-nucleus optical potentials, the neutron densities in the nuclear periphery were deduced. Assuming two-parameter Fermi distributions (2pF) describing the proton and neutron densities, the neutron rms radii were deduced for both nuclei. The difference of neutron and proton rms radii $\Delta r_{\mathit{np}}$ equal to $0.16\pm (0.02){}_{\mathrm{stat}}\pm (0.04){}_{\mathrm{syst}}$ fm for ${}^{208}\mathrm{Pb}$ and $0.14\pm (0.04){}_{\mathrm{stat}}\pm (0.04){}_{\mathrm{syst}}$ fm for ${}^{209}\mathrm{Bi}$ were determined, and the assigned systematic errors are discussed. The $\Delta r_{\mathit{np}}$ values and the deduced shapes of the neutron distributions are compared with mean field model calculations.