Pygmy and Core Polarization Dipole Modes in 206Pb: From Nuclear Structure to Stellar Nucleosynthesis | Chapter 7 | Current Perspective to Physical Science Research Vol. 9

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 (γ ⃗,γ' ) experiment at the HIγ ⃗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 8.1 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±0.2 e²  fm² and 8.3±2.0 μ_N², 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 α_D=122±10 mb/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=0.12-0.19 fm 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,γ)²⁰⁶  Pb at 30keV to be σ=130±25 mb. 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.

Author(s) Details:

A. P. Tonchev,
Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA94550, USA and Department of Physics, Duke University, Box 90308, Durham, NC27708-0308, USA.

N. Tsoneva
Institut für Theoretische Physik, Universität Gießen, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany and Extreme Light Infrastructure (ELI-NP), Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), Romania.

C. Bhatia
Department of Physics, Duke University, Box 90308, Durham, NC27708-0308, USA, Society for Applied Microwave Electronics Engineering and Research, Indian Institute of Technology, Mumbai Campus, India and Triangle Universities Nuclear Laboratory, Durham, NC27708-0308, USA.

C. W. Arnold
Bettis Atomic Power Laboratory, West Mifflin, PA 15122, USA and Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM87545, USA.

S. Goriely
Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Campus de la Plaine, CP-226, 1050 Brussels, Belgium.

S. L. Hammond
Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC27599-3255, USA and Triangle Universities Nuclear Laboratory, Durham, NC27708-0308, USA.

J. H. Kelley
Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC27599-3255, USA and Triangle Universities Nuclear Laboratory, Durham, NC27708-0308, USA.

E. Kwan
National Superconducting Cyclotron Laboratory, East Lansing, MI48824, USA.

H. Lenske
Institut für Theoretische Physik, Universität Gießen, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany.

J. Piekarewicz
Department of Physics, Florida State University, Tallahassee, FL32306-4350, USA.

R. Raut
UGC-DAE Consortium for Scientific Research, Kolkata Centre LB-8 Sector-III, Bidhannagar, Kolkata 700098, India.

G. Rusev
Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM87545, USA.

T. Shizuma
Quantum Beam Science Center, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1184, Japan.

W. Tornow
Department of Physics, Duke University, Box 90308, Durham, NC27708-0308, USA and Triangle Universities Nuclear Laboratory, Durham, NC27708-0308, USA.

Please see the link here: https://stm.bookpi.org/CPPSR-V9/article/view/14315