Structure of upper g9/2-shell nuclei and shape effect in the 94Ag isomeric states

TL;DR

This study uses a shell model to analyze the structure and shape effects of isomeric states in the upper g9/2-shell nucleus 94Ag, revealing shape coexistence and core excitation effects that influence isomerism.

Contribution

It introduces a shell model approach capable of describing the spectra and shape effects of upper g9/2-shell nuclei near N=Z, specifically explaining isomeric states in 94Ag.

Findings

Both isomeric states show large collectivity.

The 7+ state is oblate and likely a shape isomer.

Level inversion due to core excitations causes isomerism in 21+ state.

Abstract

Using a shell model which is capable of describing the spectra of upper $g_{9/2}$-shell nuclei close to the $N=Z$ line, we study the structure of two isomeric states $7^{+}$ and $21^{+}$ in the odd-odd $N=Z$ nucleus $^{94}$Ag. It is found that both isomeric states exhibit a large collectivity. The $7^{+}$ state is oblately deformed, and is suggested to be a shape isomer in nature. The $21^{+}$ state becomes isomeric because of level inversion of the $19^{+}$ and $21^{+}$ states due to core excitations across the $N=Z=50$ shell gap. Calculation of spectroscopic quadrupole moment indicates clearly an enhancement in these states due to the core excitations. However, the present shell model calculation that produces the $19^{+}$-$21^{+}$ level inversion cannot accept the large-deformation picture of Mukha {\it et al.} in Nature {\bf 439}, 298 (2006).