Quadrupole Moments of Collective Structures up to Spin $\sim$ $65\hbar$ in $^{157}$Er and $^{158}$Er: A Challenge for Understanding Triaxiality in Nuclei

TL;DR

This paper measures quadrupole moments in $^{157,158}$Er nuclei at high spins, revealing discrepancies with theoretical models and challenging current understanding of nuclear triaxial shapes.

Contribution

It provides new experimental data on quadrupole moments at high spins, questioning existing theoretical predictions of nuclear triaxiality.

Findings

Measured $Q_t$ of about 11 eb at high spins

Data inconsistent with cranked Nilsson-Strutinsky calculations

Suggests alternative triaxial shapes or rotation axes

Abstract

The transition quadrupole moments, $Q_{\rm t}$, of four weakly populated collective bands up to spin $\sim$ $65\hbar$ in $^{157,158}$Er have been measured to be ${\sim}11 {\rm eb}$ demonstrating that these sequences are associated with large deformations. However, the data are inconsistent with calculated values from cranked Nilsson-Strutinsky calculations that predict the lowest energy triaxial shape to be associated with rotation about the short principal axis. The data appear to favor either a stable triaxial shape rotating about the intermediate axis or, alternatively, a triaxial shape with larger deformation rotating about the short axis. These new results challenge the present understanding of triaxiality in nuclei.