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

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±0.2e 2fm 2 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)Pb 206 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.
 [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ;  [6] ;  [7] ;  [2] ;  [8] ;  [9] ;  [10] ;  [11] ;  [12]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Duke Univ., Durham, NC (United States)
  2. Univ. GieBen, GieBen (Germany)
  3. Indian Institute of Technology, (India); Duke Univ., Durham, NC (United States); Triangle Univ. Nuclear Lab., Durham, NC (United States)
  4. Bettis Atomic Power Lab., West Mifflin, PA (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  5. Univ. Libre de Bruxelles, Brussels (Belgium)
  6. Univ. of North Carolina, Chapel Hill, NC (United States); Triangle Univ. Nuclear Lab., Durham, NC (United States)
  7. National Superconducting Cyclotron Lab., East Lansing, MI (United States)
  8. Florida State Univ., Tallahassee, FL (United States)
  9. UGC-DAE Consortium for Scientific Research, Kolkata (India)
  10. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  11. Japan Atomic Energy Agency, Ibaraki (Japan)
  12. Duke Univ., Durham, NC (United States); Triangle Univ. Nuclear Lab., Durham, NC (United States)
Publication Date:
Report Number(s):
Journal ID: ISSN 0370-2693
Grant/Contract Number:
Published Article
Journal Name:
Physics Letters. Section B
Additional Journal Information:
Journal Volume: 773; Journal Issue: C; Journal ID: ISSN 0370-2693
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
Country of Publication:
United States
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
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1378522