Quadrupole-octupole coupling and the onset of octupole deformation in actinides

TL;DR

This study models the evolution of quadrupole and octupole shapes in actinide isotopes using a mean-field approach and boson mapping, revealing systematic transitions to octupole deformation.

Contribution

It introduces a combined mean-field and boson model approach to analyze octupole deformation in actinides, providing detailed spectroscopic predictions.

Findings

Identification of systematic transition from spherical to octupole-deformed shapes.

Prediction of low-energy negative-parity spectra consistent with experimental data.

Quantitative estimates of octupole transition rates and deformation parameters.

Abstract

The evolution of quadrupole and octupole collectivity and their coupling is investigated in a series of even-even isotopes of the actinide Ra, Th, U, Pu, Cm, and Cf with neutron number in the interval $130\leqslant N\leqslant 150$. The Hartree-Fock-Bogoliubov approximation, based on the parametrization D1M of the Gogny energy density functional, is employed to generate potential energy surfaces depending upon the axially-symmetric quadrupole and octupole shape degrees of freedom. The mean-field energy surface is then mapped onto the expectation value of the $sdf$ interacting-boson-model Hamiltonian in the boson condensate state as to determine the strength parameters of the boson Hamiltonian. Spectroscopic properties related to the octupole degree of freedom are produced by diagonalizing the mapped Hamiltonian. Calculated low-energy negative-parity spectra, $B(E3;3^{-}_{1}\to 0^{+}_{1})$ reduced transition rates, and effective octupole deformation suggest that the transition from nearly spherical to stable octupole-deformed, and to octupole vibrational states occurs systematically in the actinide region.