Abstract:

We study whether the neutron skin thickness Δrnp of 208Pb originates from the bulk or from the surface of the nucleon density distributions, according to the meanfield models of nuclear structure, and find that it depends on the stiffness of the nuclear symmetry energy. The bulk contribution to Δrnp arises from an extended sharp radius of neutrons, whereas the surface contribution arises from different widths of the neutron and proton surfaces. Nuclear models where the symmetry energy is stiff, as typical of relativistic models, predict a bulk contribution in Δrnp of 208Pb about twice as large as the surface contribution. In contrast, models with a soft symmetry energy like common nonrelativistic models predict that Δrnp of 208Pb is divided similarly into bulk and surface parts. Indeed, if the symmetry energy is supersoft, the surface contribution becomes dominant. We note that the linear correlation of Δrnp of 208Pb with the density derivative of the nuclear symmetry energy arises from the bulk part of Δrnp. We also note that most models predict a mixedtype (between halo and skin) neutron distribution for 208Pb. Although the halotype limit is actually found in the models with a supersoft symmetry energy, the skintype limit is not supported by any meanfield model. Finally, we compute parityviolating electron scattering in the conditions of the 208Pb parity radius experiment (PREX) and obtain a pocket formula for the parityviolating asymmetry in terms of the parameters that characterize the shape of the 208Pb nucleon densities. 