First Evidence of Axial Shape Asymmetry and Configuration Coexistence in ${}^{74}$Zn: Suggestion for a Northern Extension of the $N=40$ Island of Inversion

Abstract

The excited states of $N=44$ ${}^{74}$Zn were investigated via $\gamma$-ray spectroscopy following ${}^{74}$Cu $\beta$ decay. By exploiting $\gamma$-$\gamma$ angular correlation analysis, the $2_2^{+}$, $3_1^{+}$, $0_2^{+}$ and $2_3^{+}$ states in ${}^{74}$Zn were firmly established. The $\gamma$-ray branching and $E2/M1$ mixing ratios for transitions de-exciting the $2_2^{+}$, $3_1^{+}$ and $2_3^{+}$ states were measured, allowing for the extraction of relative $B(E2)$ values. In particular, the $2_3^{+}\to 0_2^{+}$ and $2_3^{+}\to 4_1^{+}$ transitions were observed for the first time. The results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the $N=40$ gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize ${}^{74}$Zn in its ground state. Furthermore, an excited $K=0$ band with a significantly larger softness in its shape is identified. A shore of the $N=40$ ``island of inversion'' appears to manifest above $Z=26$, previously thought as its northern limit in the chart of the nuclides.

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Cited by in corpus (4)

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• Large scale shell model calculation for collectivity in nuclei beyond 78Ni
• Commissioning and Full Realization of the PLASEN System at BRIF