Broken axial symmetry as essential feature for a consistent modelling of various observables in heavy nuclei

TL;DR

This paper demonstrates that breaking axial symmetry in nuclear models simplifies the description of heavy nuclei, improves agreement with experimental data, and aligns with advanced theoretical calculations, without needing extra fit parameters.

Contribution

It introduces a triaxial parametrization based on broken axial symmetry that enhances modeling accuracy of heavy nuclei observables without additional fitting parameters.

Findings

Triaxiality provides a simple heuristic for yrast sequences in heavy nuclei.

Improved predictions for electric dipole strength and neutron capture rates.

Alignment with HFB and MC-shell model calculations for stable nuclei.

Abstract

Although most nuclear spectroscopy as well as atomic hyperfine structure data do not deliver accurate information on nuclear axiality the ad-hoc assumption of symmetry about one axis found widespread use in nuclear model calculations. In the theoretical interpretation of nuclear properties as well as in the analysis of experimental data triaxiality was considered - if at all - only for some, often exotic, nuclides. A breaking of axial symmetry combined to a spin-independent moment of inertia results in a surprisingly simple heuristic triaxial parametrization of the yrast sequence in all heavy nuclei, including well deformed ones. No additional fit parameters are needed in detailed studies of the mass and charge dependence of the electric dipole strength in the range of and outside of giant dipole resonances. Allowing triaxiality also avoids the introduction of an arbitrary level density parameter \=a to fit the accurate values observed in n-capture experiments and \=a can be taken from nuclear matter studies. A combination of this value to the yrast energies no longer based on axiality and the related I(I+1) rule results in agreement to data independent of spin. And predictions for radiative neutron capture as derived on the basis of non-axiality are improved as well. The experimentally favoured broken axial symmetry is in accord to HFB and MC-shell model calculations already for nuclei in the valley of stability.