Fragmentation and systematics of the Pygmy Dipole Resonance in the stable N=82 isotones

TL;DR

This study systematically investigates the fragmentation and systematics of the pygmy dipole resonance in stable N=82 isotones using nuclear resonance fluorescence, revealing how fragmentation and strength depend on neutron-to-proton ratio and comparing results with microscopic models.

Contribution

It provides the first comprehensive systematic survey of PDR fragmentation in all stable N=82 isotones and compares experimental data with advanced microscopic calculations.

Findings

Fragmentation decreases towards proton-deficient isotones.

Total PDR strength increases with neutron-to-proton ratio.

Experimental data agrees with quasi-particle phonon model calculations.

Abstract

The low-lying electric dipole (E1) strength in the semi-magic nucleus 136Xe has been measured which finalizes the systematic survey to investigate the so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with the method of nuclear resonance fluorescence using real photons in the entrance channel. In all cases, a fragmented resonance-like structure of E1 strength is observed in the energy region 5 MeV to 8 MeV. An analysis of the fragmentation of the strength reveals that the degree of fragmentation decreases towards the proton-deficient isotones while the total integrated strength increases indicating a dependence of the total strength on the neutron-to-proton ratio. The experimental results are compared to microscopic calculations within the quasi-particle phonon model (QPM). The calculation includes complex configurations of up to three phonons and is able to reproduce also the fragmentation of the E1 strength which allows to draw conclusions on the damping of the PDR. Calculations and experimental data are in good agreement in the degree of fragmentation and also in the integrated strength if the sensitivity limit of the experiments is taken into account.