Is Seniority a Partial Dynamic Symmetry in the First $\nu g_{9/2}$ Shell?

TL;DR

This study investigates the seniority quantum number in the first $g_{9/2}$ shell of Ni isotopes, revealing partial preservation of seniority symmetry and its breakdown at higher spins through experimental and shell-model comparisons.

Contribution

It provides the first complete experimental picture of low-lying states in $^{72}$Ni and $^{74}$Ni up to $8^+$, and compares these with shell-model predictions to assess seniority preservation.

Findings

Disappearance of seniority isomerism explained by lowering of $(6^+)$ levels.

Restoration of seniority ordering observed in $^{74}$Ni supports spherical regime dominance.

Experimental data aligns with shell-model predictions up to certain spins.

Abstract

The low-lying structures of the midshell $\nu g_{9/2}$ Ni isotopes $^{72}$Ni and $^{74}$Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following $\beta$ decay of the mother nuclei $^{72}$Co and $^{74}$Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first $(8^+)$ levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron $g_{9/2}$ shell. We find that the disappearance of the seniority isomerism in the $(8^+_1)$ states can be explained by a lowering of the seniority-four $(6^+)$ levels as predicted years ago. For $^{74}$Ni, the internal de-excitation pattern of the newly observed $(6^+_2)$ state supports a restoration of the normal seniority ordering up to spin $J=4$. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near $^{78}$Ni.