Pygmy and core polarization dipole modes in 206Pb: Connecting nuclear structure to stellar nucleosynthesis

A high-resolution study of the electromagnetic response of Pb-206 below the neutron separation energy is performed using a ((gamma) over right arrow,gamma ') experiment at the HI (gamma) over right arrow .S facility. Nuclear resonance fluorescence with 100% linearly polarized photon beams is used to measure spins, parities, branching ratios, and decay widths of excited states in Pb-206 from 4.9 to 8.1 MeV. The extracted Sigma B(E1)up arrow and Sigma B(M1) up arrow values for the total electric and magnetic dipole strength below the neutron separation energy are 0.9 +/- 0.2 e(2)fm(2) and 8.3 +/- 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 pygmy dipole resonance from both the tail of the giant dipole resonance and multi-phonon excitations. Combined with earlier photonuclear experiments above the neutron separation energy, one extracts a value for the electric dipole polarizability of Pb-206 of alpha(D)= 122 +/- 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-skin(206)= 0.12-0.19fm and a corresponding range for the slope of the symmetry energy of L = 48-60MeV. This newly obtained information is also used to estimate the Maxwellian-averaged radiative cross section Pb-205(n, gamma) Pb-206 at 30 keV to be sigma= 130 +/- 25mb. The astrophysical impact of this measurement-on both the s-process in stellar nucleosynthesis and on the equation of state of neutron-rich matter-is discussed. Published by Elsevier B.V.