Neutron density distributions from antiprotonic 208Pb and 209Bi atoms

The x-ray cascade from antiprotonic atoms was studied for 208 Pb and 209 Bi. Widths and shifts of the levels due to the strong interaction were determined. Using modem 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 Δr nP equal to 0.16 ± (0.02) stat ± (0.04) syst fm for 208 Pb and 0.14 ± (0.04) stat ± (0.04) syst fm for 209 Bi were determined, and the assigned systematic errors are discussed. The Δr nP values and the deduced shapes of the neutron distributions are compared with mean field model calculations.