Pygmy dipole resonance in 208Pb

TL;DR

This paper demonstrates that high-energy proton scattering at forward angles can effectively study the pygmy dipole resonance in 208Pb, enabling extraction of electromagnetic transition strengths and resonance energies.

Contribution

It introduces a novel method combining proton scattering and multipole decomposition analysis to study the PDR in 208Pb, providing new experimental insights.

Findings

Successful extraction of B(E1) strength distribution up to 9 MeV

First experimental determination of PDR energy centroid

Sensitivity of Coulomb-nuclear interference to E1 structure

Abstract

Scattering of protons of several hundred MeV is a promising new spectroscopic tool for the study of electric dipole strength in nuclei. A case study of 208Pb shows that at very forward angles J^pi = 1- states are strongly populated via Coulomb excitation. A separation from nuclear excitation of other modes is achieved by a multipole decomposition analysis of the experimental cross sections based on theoretical angular distributions calculated within the quasiparticle-phonon model. The B(E1) transition strength distribution is extracted for excitation energies up to 9 MeV, i.e., in the region of the so-called pygmy dipole resonance (PDR). The Coulomb-nuclear interference shows sensitivity to the underlying structure of the E1 transitions, which allows for the first time an experimental extraction of the electromagnetic transition strength and the energy centroid of the PDR.