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

The excited states of N = 44 74Zn were investigated via gamma-ray spectroscopy following 74Cu beta decay. By exploiting gamma-gamma angular correlation analysis, the 2 thorn 2 , 3 thorn 1 , 0 thorn 2 , and 2 thorn 3 states in 74Zn were firmly established. The gamma-ray branching and E2/M 1 mixing ratios for transitions deexciting the 2 thorn 2 , 3 thorn 1 , and 2 thorn 3 states were measured, allowing for the extraction of relative BoE2 thorn values. In particular, the 2 thorn 3 -0 thorn 2 and 2 thorn 3 -4 thorn 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 74Zn 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.