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

Journal Article · Sun Oct 01 00:00:00 EDT 2017 · Physics Letters B

DOI:https://doi.org/10.1016/j.physletb.2017.07.062· OSTI ID:1373624

Tonchev, A. P.; Tsoneva, N.; Bhatia, C.; Arnold, C. W.; Goriely, S.; Hammond, S. L.; Kelley, J. H.; Kwan, E.; Lenske, H.; Piekarewicz, J.; Raut, R.; Rusev, G.; Shizuma, T.; Tornow, W.

A high-resolution study of the electromagnetic response of ²⁰⁶Pb below the neutron separation energy is performed using a (γ→,γ') experiment at the HIγ→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 ²⁰⁶Pb from 4.9 to 8.1 MeV. The extracted ΣB(E1)↑ and ΣB(M1)↑ values for the total electric and magnetic dipole strength below the neutron separation energy are 0.9±0.2e²fm² and 8.3±2.0μ$$2\atop{N}$$, 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 ²⁰⁶Pb of α_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$$206\atop{skin}$$=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 ₂₀₅Pb(n,γ)Pb₂₀₆ at 30 keV to be σ=130±25mb. In conclusion, 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.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: 16-ERD-022; FG02-97ER41033; FG02-97ER41041; FG02-97ER41042; FG52-06NA26155; FD05-92ER40750; AC52-07NA27344
• OSTI ID: 1373624
• Alternate ID(s): OSTI ID: 1378522
• Report Number(s): LLNL-JRNL-716577; S0370269317306184; PII: S0370269317306184
• Journal Information: Physics Letters B, Journal Name: Physics Letters B Vol. 773 Journal Issue: C; ISSN 0370-2693
• Publisher: Elsevier
• Country of Publication: Netherlands
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
79 ASTRONOMY AND ASTROPHYSICS
206Pb
Pygmy dipole resonance
Giant dipole resonance
Skin thickness and dipole polarizability
Pb205(n
γ)Pb206
reaction cross section