COMPOSITION OF THE NUCLEAR PERIPHERY FROM ANTIPROTON ABSORPTION

Thirteen targets with mass numbers from 58 to 238 were irradiated with the antiproton beam from the Low Energy Antiproton Ring facility at C ERN leading to the formation of antiprotonic atoms of these heavy elem ents. The antiproton capture at the end of an atomic cascade results i n the production of more or less excited residual nuclei. The targets were selected with the criterion that both reaction products with one nucleon less than the proton and neutron number of the target be radio active. The yield of these radioactive products after stopped-antiprot on annihilation was determined using gamma-ray spectroscopy techniques . This yield is related to the proton and neutron density in the targe t nucleus at a radial distance corresponding to the antiproton annihil ation site. The experimental data clearly indicate the existence of a neutron-rich nuclear periphery, a ''neutron halo,'' strongly correlate d with the target neutron separation energy B-n and observed for targe ts with B-n<10 MeV. For two-target nuclei Cd-106 and Sm-144, with larg er neutron binding energies, a proton-rich nuclear periphery was obser ved. Most of the experimental data are in reasonable agreement with ca lculations based on current antiproton-nucleus and pion-nucleus intera ction potentials and on nuclear densities deduced with the help of the Hartree-Fock-Bogoliubov approach. This approach was, however, unable to account for the Cd-106 and Sm-144 results.