Pygmy and Core Polarization Dipole Modes in 206Pb: From Nuclear Structure to Stellar Nucleosynthesis

The work reported here highlights the vital and increasingly important role that measurements of exotic modes of excitation in neutron-rich nuclei play in determining observables of critical astrophysical significance. A high-resolution study of the electromagnetic response of \(^{206}\)Pb below the neutron separation energy is performed using a \(\vec\gamma\),\(\gamma\)\(\prime\)) experiment at the HI\(\vec\gamma\)S facility. The Pygmy Dipole Resonance (PDR)—the emergence of low-energy electric dipole strength with neutron excess—has motivated a great deal of experimental and theoretical effort. Nuclear resonance fluorescence (NRF) with 100% linearly polarized photon beams is used to measure spins, parities, branching ratios, and decay widths of excited states in \(^{206}\)Pb from 4.9 to 8.1MeV. The extracted \(\Sigma\)B (E1) \(\uparrow\) and \(\Sigma\)B (M1) \(\uparrow\) values for the total electric and magnetic dipole strength below the neutron separation energy are 0.9 \(\pm\) 0.2e2fm2 and 8.3 \(\pm\) 2.0 \(\mu^{2}_{N}\), respectively. These measurements are found to be in very good agreement with the predictions from an energy-density functional (EDF) plus quasiparticle phonon model (QPM). Such a detailed theoretical analysis allows to separate the PDR from both the tail of the Giant Dipole Resonance (GDR) and multi-phonon excitations. The EDF+QPM theory successfully reproduces the low-energy M1 spectral distribution, suggesting that it is mostly due to spin-flip excitations. In combination with relativistic EDFs that are accurately calibrated to ground-state properties of finite nuclei, estimates for the neutron-skin thickness in both 206Pb and 208Pb are provided, with the latter consistent with some recent analyses. Combined with earlier photonuclear experiments above the neutron separation energy, one extracts a value for the electric dipole polarizability of \(^{206}\)Pb of \(\alpha\)D =122\(\pm\) 10mb/MeV. When compared to predictions from both the EDF+QPM and accurately calibrated relativistic EDFs, one deduces a range for the neutron-skin thickness of \(R^{206}_{skin}\) =0.12 - 0.19fm and a corresponding range for the slope of the symmetry energy of L= 48 - 60 MeV. This newly obtained information is also used to estimate the Maxwellian-averaged radiative cross-section \(^{205}\)Pb(n,\(\gamma\))\(^{206}\)Pb at 30keV to be \(\sigma\) = 130 \(\pm\) 25 mb. The astrophysical impact of this measurement on both the s-process in stellar nucleosynthesis and on the equation of the state of neutron-rich matter is discussed.