Impact of octupole deformation on the nuclear electromagnetic response

TL;DR

This study examines how octupole deformation influences nuclear electromagnetic responses, revealing modest effects on transition strengths and highlighting the significance of parity-breaking solutions.

Contribution

It provides a detailed analysis of octupole deformation effects on nuclear resonances using advanced theoretical models and compares parity-conserving and parity-breaking ground states.

Findings

Octupole deformation has a modest impact on transition strengths.

$M1$ transition strengths are more affected at lower energies (0-8 MeV).

Significant contribution of the rotational Nambu-Goldstone mode to $E3$ transition strength.

Abstract

Background: Properties of giant dipole resonances, along with other nuclear resonances, provide valuable tools for refining theoretical models as they reflect collective features of nuclear matter. Among such collective phenomena is octupole deformation, whose impact on resonance features, however, is less studied. Purpose: Investigate the effect of reflection-symmetry-breaking octupole deformation on electric and magnetic transition strengths in atomic nuclei. Methods: Calculations were performed using linear response theory with the iterative finite amplitude method to solve quasiparticle random phase approximation-type equations. Underlying ground-state solutions were obtained within the framework of axially symmetric Skyrme-Hartree-Fock-Bogoliubov (HFB) using three different Skyrme functionals. Results: Electric and magnetic multipole responses were calculated for octupole-deformed even-even Rn, Ra, Th, U, Pu, and Cm isotopes. Calculations were performed on top of two distinct deformed ground-state solutions: one constrained to conserve parity, and the other allowing parity breaking. Sum rules were calculated from $M1$ transition strengths and compared with the expected correlations to certain ground-state properties. Conclusions: Based on our results, the octupole deformation has only a modest effect on the transition strengths in the resonances. In turn, $M1$ transition strengths have a greater effect at lower energies ($0\,\text{--}\,8\,\text{MeV}$), which encourages further investigation. Isoscalar $E3$ transition strength was confirmed to have a significant contribution from the rotational Nambu--Goldstone mode in the parity-breaking HFB solution, and thus, removing it was found necessary.