Composition of the nuclear periphery from antiproton absorption

Abstract

Thirteen targets with mass numbers from 58 to 238 were irradiated with the antiproton beam from the Low Energy Antiproton Ring facility at CERN leading to the formation of antiprotonic atoms of these heavy elements. The antiproton capture at the end of atomic cascade results in 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 are radioactive. The yield of these radioactive products after stopped-antiproton annihilation was determined using gamma-ray spectroscopy techniques. This yield is related to the proton and neutron density in the target nucleus at radial distance corresponding to the antiproton annihilation site. The experimental data clearly indicate the existence of a neutron rich nuclear periphery, a "neutron halo", strongly correlated with the target neutron separation energy Bn, and observed for targets with Bn < 10 MeV. For two target nuclei, 106 Cd and 144 Sm, with larger neutron binding energies a proton rich nuclear periphery was observed. Most of the experimental data are in reasonable agreement with calculations based on current antiproton-nucleus and pion-nucleus interaction potentials and on nuclear densities deduced with the help of the Hartree-Fock-Bogoliubov approach. This approach was, however, unable to account for the 106 Cd and 144 Sm results.

Neutron density distributions from antiprotonicPb208andBi209

“…This yields r π = c ch + 1.35 fm. [3,4,66] as a function of the target-nucleus neutron binding energy. For 208 Pb, the neutron binding energy is 7.4 MeV and the interpolated halo factor is 2.8 ± 0.4.…”

“…First, using the so-called radiochemical method, we investigated [1][2][3][4] the ratios of peripheral neutron to proton densities at distances around 2.5 fm larger than the nuclear charge half-density radius [5]. The method involved measuring the yield of radioactive nuclei having one proton or one neutron less than the target nucleus, produced after antiproton capture, cascade, and annihilation in the target antiprotonic atom.…”

Systematics of nucleon density distributions and neutron skin of nuclei

“…For instance, the elastic scattering processes of proton [9,11,12, and 13] and α-particle projectiles [14], and the experiments involving giant dipole and spin-dipole resonances [15,16]. Also, the radiochemical and x-ray data of antiprotonic atoms, such as the atomic level shifts and level widths, are usually proposed to this aim [17,18,19,20,21,22]. The precise analysis of such experimental data is of special interest not only to determine the density distribution and the neutron skin but also to investigate the different conditions influencing their bulk and surface parts.…”

Antiprotonic atoms X rays

“…In the second method the yield of the radioactive antiproton annihilation residues one mass unit lighter that the target was measured. The yield of products with one neutron less than the target neutron number N t (annihilation on a neutron) and those with one proton less than the target proton number Z t (annihilation on a proton) were determined [5,6,7]. The halo factor was calculated using these yields [5].…”