First Evidence of Axial Shape Asymmetry and Configuration Coexistence in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Zn</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>74</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>: Suggestion for a Northern Extension of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>40</mml:mn></mml:math> Island of Inversion
M. Rocchini, P. E. Garrett, M. Zielińska, S. M. Lenzi, D. D. Dao, F. Nowacki, V. Bildstein, A. D. MacLean, B. Olaizola, Z. T. Ahmed, C. Andreoiu, A. Mohan Babu, G. C. Ball, S. S. Bhattacharjee, H. Bidaman, Chloe M Cheng, R. Coleman, I. Dillmann, A. B. Garnsworthy, S. A. Gillespie, C. J. Griffin, G. F. Grinyer, G. Hackman, M. Hanley, A. Illana, S. Jones, A. T. Laffoley, K. G. Leach, R. S. Lubna, J. R. McAfee, C. R. Natzke, S. Pannu, C. Paxman, C. Porzio, A. J. Radich, M. M. Rajabali, F. Sarazin, K. Schwarz, S.C. Shadrick, S. Sharma, J. S. Suh, C. E. Svensson, D. Yates, T. Zidar
Abstract
The excited states of $N=44$ $^{74}\mathrm{Zn}$ were investigated via $\ensuremath{\gamma}$-ray spectroscopy following $^{74}\mathrm{Cu}$ $\ensuremath{\beta}$ decay. By exploiting $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ angular correlation analysis, the ${2}_{2}^{+}$, ${3}_{1}^{+}$, ${0}_{2}^{+}$, and ${2}_{3}^{+}$ states in $^{74}\mathrm{Zn}$ were firmly established. The $\ensuremath{\gamma}$-ray branching and $E2/M1$ mixing ratios for transitions deexciting the ${2}_{2}^{+}$, ${3}_{1}^{+}$, and ${2}_{3}^{+}$ states were measured, allowing for the extraction of relative $B(E2)$ values. In particular, the ${2}_{3}^{+}\ensuremath{\rightarrow}{0}_{2}^{+}$ and ${2}_{3}^{+}\ensuremath{\rightarrow}{4}_{1}^{+}$ 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 $^{74}\mathrm{Zn}$ 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.