Breaking of axial symmetry in excited heavy nuclei as identified in Giant Dipole Resonance data

TL;DR

This paper critically analyzes photon strength functions in heavy nuclei, using a triple Lorentzian model for the giant dipole resonance, and discusses the implications of axial symmetry breaking on nuclear structure predictions.

Contribution

It introduces a global triple Lorentzian parameterization of GDR shapes that accounts for axial symmetry breaking in heavy nuclei, improving predictive power for photon strength functions.

Findings

The TLO model fits photon strength data well across energies.

Non-GDR modes contribute to low-energy photon strength.

Axial symmetry breaking is supported in stable heavy nuclei.

Abstract

A recent theoretical prediction of a breaking of axial symmetry in quasi all heavy nuclei is confronted to a new critical analysis of photon strength functions of nuclei in the valley of stability. For the photon strength in the isovector giant dipole resonance (IVGDR) regime a parameterization of GDR shapes by the sum of three Lorentzians (TLO) is extrapolated to energies below and above the IVGDR. The impact of non-GDR modes adding to the low energy slope of photon strength is discussed including recent data on photon scattering and other radiative processes. These are shown to be concentrated in energy regions where various model calculations predict intermediate collective strength; thus they are obviously separate from the IVGDR tail. The triple Lorentzian (TLO) ansatz for giant dipole resonances is normalized in accordance to the dipole sum rule. The nuclear droplet model with surface dissipation accounts well for positions and widths without local, nuclide specific, parameters. Very few and only global parameters are needed when a breaking of axial symmetry already in the valley of stability is admitted and hence a reliable prediction for electric dipole strength functions also outside of it is expected.