Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in 208Pb


M. Spieker, A. Heusler, B. A. Brown , T. Faestermann, R. Hertenberger, G. Potel, M. Scheck, N. Tsoneva, M. Weinert, H.-F. Wirth, and A. Zilges

The first comprehensive investigation of the single-particle character of the pygmy dipole resonance (PDR) in 208Pb on the basis of new experimental data and their theoretical explanation is discussed in Physical Review Letters [DOI: 10.1103/PhysRevLett.125.102503].

The new findings come from an international collaboration led by nuclear experimentalist Dr. M. Spieker (Florida State University, USA). The novel data were collected from (d,p) and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany.
The theoretical work was carried out by nuclear theorists Dr. N. Tsoneva (ELI-NP), a recognized expert in energy-density functional and quasiparticle-phonon model theory (EDF+QPM), and Prof. B. A. Brown (Michigan State University, USA), a world leader in large-scale nuclear shell model (LSSM).

208Pb is a heavy atomic nucleus with excess neutrons, which leads to the formation of a neutron skin. The phenomenon can affect the nuclear response to external electromagnetic and hadronic fields from such nuclei. It is observed that nuclear dipole excitations, especially at low energies, react very sensitively to the presence of a neutron skin and can resemble a resonance-like structure which corresponds to the vibrations of the neutron skin. This oscillation mode is known as PDR. In addition to its isospin structure, the degree of collectivity of the PDR is still being discussed. In this letter, a detailed, high-resolution (d, p) experimental study of the PDR in 208Pb is presented and supplemented with available experimental data to discuss the microscopic structure of the PDR and its impact on experimental observables by comparing it to the state-of-the-art theoretical models: EDF+QPM and LSSM. The neutron one-particle-one-hole (1p-1h) configurations that contribute to the formation of the PDR are accessed from (d, p) data up to the proton separation energy Sp and for a limited number of states from the results of resonant proton scattering via isobaric analog resonances (p, p')IAR, which probes components that could not be populated in the selective one-neutron transfer reaction. In addition, the experimental observations are compared to the large suite of complementary, experimental data available for 208Pb and establish (d,p) as an additional, valuable, experimental probe to study the PDR and its collectivity, i.e. the coherence between the different 1p-1h contributions. Unprecedented access to the theoretical wave functions demonstrating the 1p-1h neutron origin of the PDR in 208Pb has been achieved. The comparison of LSSM and EDF + QPM calculations shows that both models were able to account the main features of the (d, p) data. The present studies of the PDR in 208Pb will support ELI-Gamma-Above-Neutron-Threshold (ELIGANT) day-one experiments at ELI-NP which target the GDR and the PDR ground state γ-decays, as well as studies of multi-step γ-decays through low-lying states. With the unique possibilities of the VEGA system, the ELI-NP can deliver high-resolution data stemming from photon-induced excitations and reactions. In this regard, further studying the PDR at ELI-NP will provide new insights into the dynamics of isospin-asymmetric nuclear matter and the properties of neutron stars.

(a) Angle-integrated (d,p) cross sections σ(d,p). (b) CLJlj amplitudes from (p,p')IAR. (c) σ(d,p). predicted by combining LSSM spectroscopic factors with DWBA calculations. (d) Decomposition of the LSSM wave functions into neutron 1p-1h components relative to the total wave function ψ total. (e), (f) same as (c), (d) but for EDF + QPM.