Probing the structure of pygmy dipole resonance with its gamma decay

TL;DR

This study investigates the gamma decay properties of pygmy dipole resonance in lead-208 to understand its isospin nature and collectivity, revealing its predominantly isoscalar character through suppressed E1 decay.

Contribution

It introduces a detailed analysis of PDR gamma decay using the Skyrme PVC model, highlighting its isoscalar nature and complex configuration components.

Findings

PDR gamma decay to the 2+ state is strongly suppressed compared to IVGDR.

PDR has a smaller complex configuration component than IVGDR and ISGQR.

The decay analysis reveals the predominantly isoscalar character of PDR.

Abstract

The isospin properties and the collectivity of the pygmy dipole resonance (PDR) are long-standing open questions in nuclear structure studies. To answer these questions, the $\gamma$-decay of PDR states in $^{208}$Pb to the low-lying $2_{1}^{+}$ state is investigated using the Skyrme particle-vibration coupling (PVC) model. It is found that the $E1$ $\gamma$ decay from the PDR states to the low-lying $2_{1}^+$ state is strongly suppressed compared to that from the isovector giant dipole resonance (IVGDR), which reveals the predominantly isoscalar character of PDR. A detailed decomposition of the decay diagrams of the amplitudes of the processes contributing to the decay demonstrates the non-negligible presence of the 1 particle-1 hole configurations coupled to the $2_1^{+}$ phonon in the PDR wave function. Furthermore, we give a quantitative way to identify the components of complex-configurations in the wave function, and it is found that such component in PDR is smaller than that in IVGDR and much smaller than that in isoscalar giant quadrupole resonance (ISGQR).