Composition of the nuclear periphery from antiproton absorption using short-lived residual nuclei

The neutron-to-proton density ratio at the nuclear periphery was determined for six targets from the yield of nuclei with mass (At21) generated by antiproton annihilation on targets with mass number At . A new setup allowed us to measure this yield for nuclei with half-lives down to 6 s. The experiment confirmed the negative correlation between the peripheral neutron-to-proton density ratio and the neutron binding energy, which had been previously found. Results of Hartree-Fock-Bogoliubov and relativistic Hartree-Bogoliubov calculations show good agreement for a large number of nuclei. By comparing the measured values with those derived from Hartree-Fock-Bogoliubov calculations, the ratio R of the imaginary parts of the effective scattering lengths of antiprotons on bound neutrons and protons was deduced to be R50.69(4). For the first time yields of several excited states of some (At21) nuclei have been investigated. @S0556-2813~99!04410-6#