Narrow Results

The soft dipole modes in neutron-rich even–even Ni-isotopes are investigated in the quasiparticle relativistic random phase approximation. We study the evolution of strengths distribution, centroid energies of dipole excitation in low-lying and normal GDR regions with the increase of the neutron excess. It is found in the present study that the centroid energies of the soft dipole strengths strongly depend on the thickness of neutron skin along with the neutron-rich even–even Ni-isotopes.

The neutron skin thickness of ⁴⁸Ca was deduced from the interaction cross-section by adopting a microscopic optical potential. The optical potential used was constructed by folding a chiral $g$ matrix and the Skyrme mean-field densities renormalized by considering the information of the interaction cross-section. The result was $R_{\mathrm{\text{skin}}}=0.139\pm 0.058$ fm.

In heavy nuclei the spatial distribution of protons and neutrons is different. At CERN SPS energies production of π⁺ and π^- differs for pp, pn, np and nn scattering. These two facts lead to an impact parameter dependence of the π⁺ to π^- ratio in ²⁰⁸Pb+²⁰⁸Pb collisions. A recent experiment at CERN seems to confirm qualitatively these predictions. It may open a possibility for determination of neutron density distribution in nuclei.

We analyze theoretically the neutron skin thickness in nuclei and its correlation with the symmetry energy by using semiclassical and mean field approaches together with nuclear effective interactions. Semiclassical approaches reveal that the neutron skin thickness in nuclei is formed by a combination of bulk and surface contributions. To investigate the neutron skin thickness predicted by mean field models, we fit the corresponding densities by two-parameter Fermi distributions. Using these parametrized densities, we study the neutron skin thickness as well as its bulk and surface contributions in ²⁰⁸Pb and in Zr isotopes, where the influence of shell effects along the isotopic chain is discussed.

The emission of spectator neutrons and protons in ultracentral collisions of Bi with the same (Bi) or lighter (W, Cu) target nuclei has been simulated using the Abrasion-Ablation Monte Carlo for Colliders model with MST clustering (AAMCC-MST). It was shown that the detection of forward neutrons and protons in such collisions allows the analysis of the $n∕p$ ratio at the nuclear periphery. The cross-sections to produce a given number of spectator neutrons $N_n$, the average number of spectator neutrons $〈N_n〉$ and protons $〈N_p〉$ in ultracentral collisions were calculated with and without consideration of the neutron skin in Bi. The ratio $〈N_n∕N_p〉$ was found to be particularly sensitive to the presence of neutron skin in Bi–W collisions, providing a potential probe of the enrichment of the nuclear surface by neutrons. However, this sensitivity disappears in Bi–Cu collisions due to the involvement of inner nuclear layers to the spectator emission.

We discuss to what extent information on ground-state properties of finite nuclei (energies and radii) can be used to obtain constraints on the symmetry energy in nuclear matter and its dependence on the density. The present approach is based upon a generalized Weizsäcker formula for ground-state energies. In particular effects from the Wigner energy and shell structure on the symmetry energy are investigated. Data on the neutron skin is used as an additional source of information.

I discuss advances in nuclear structure theory and the implications for the study of fundamental symmetries in nuclei. Recent results for the properties of the neutron skin in heavy nuclei are summarized.

We show in the Skyrme-Hartree-Fock approach that unambiguous correlations exist between observables of finite nuclei and nuclear matter properties. Using this correlation analysis to existing data on the neutron skin thickness of Sn isotopes, we find important constraints on the value E_sym(ρ0) and density slope L of the nuclear symmetry energy at saturation density. Combining these constraints with those from recent analyses of isospin diffusion and double neutron/proton ratio in heavy ion collisions leads to a value of L = 58 ± 18 MeV approximately independent of E_sym(ρ0).

Small-angle polarized proton scattering including 0° on ²⁰⁸Pb has been studied at the RCNP cyclotron with high energy resolution of the order 25 — 30 keV (FWHM). The complete E1 strength distribution from 5 to 20 MeV could be extracted from the data. The total E1 polarizability as well as the properties of the pygmy dipole resonance can be extracted with high precision providing important experimental constraints on the neutron skin thickness in ²⁰⁸Pb and the symmetry energy of neutron matter.

Small-angle polarized proton scattering including 0° off ²⁰⁸Pb has been studied at the RCNP cyclotron with high energy resolution of the order of 25 keV (FWHM). The complete E1 strength distribution from 5 to 20 MeV could be extracted from the data and is found to agree well with available data. New E1 strength is found in the energy region above threshold inacessible in previous experiments. The total E1 polarizability as well as the properties of the pygmy dipole resonance could be determined with high precision providing important experimental constraints on the neutron skin thickness in ²⁰⁸Pb and the symmetry energy of neutron-rich matter. Additionally, information on the spin-M1 strength in ²⁰⁸Pb was obtained. Assuming dominance of the central spin-isospinflip part of the effective proton-nucleus interaction, the B(M1) transition strength can be derived. It corresponds well with data from electromagnetic probes indicating that the reaction can provide information on the poorly known spin-M1 resonance in heavy nuclei.