Triaxiality in selenium-76

Abstract

Background: Selenium and germanium nuclei are associated with both triaxiality and shape coexistence. The relative influence of these deformation effects on the low-lying nuclear structure remains the subject of much discussion with additional attention drawn to $^{76}\mathrm{Se}$ and $^{76}\mathrm{Ge}$ due to the potential for the observation of neutrinoless double-$\ensuremath{\beta}$ decay.Purpose: Experimental observables related to the deformation of $^{76}\mathrm{Se}$ are lacking in precision. The purpose of the present paper is to provide electric quadrupole matrix elements with improved precision in order to determine the deformation of low-lying states in a model-independent manner.Methods: Sub-barrier Coulomb excitation was employed at the reaccelerated beam facility of the National Superconducting Cyclotron Laboratory using the JANUS setup. Using this method nineteen $E2$ matrix elements were extracted.Results: Extracted matrix elements agree within uncertainties with those in the literature but with improved precision. Through both a comparison with geometric models and a model-independent evaluation of $E2$ matrix elements using rotational invariants, the ground state of $^{76}\mathrm{Se}$ is best described as having a significant triaxial component, but is not maximally triaxially deformed.Conclusions: Selenium-76 exhibits a significant degree of triaxiality in its ground state. A detailed comparison with configuration-interaction calculations indicates that this can be well reproduced theoretically.