In recent decades, rare-isotope facilities have enabled the study of short-lived, neutron-rich nuclei. Their measured properties indicate that shell structure changes in the regime of unbalanced neutron-to-proton ratios compared with that of stable nuclei. In the so-called islands of inversion in the nuclear chart—around the neutron-rich nuclei ³²Mg, ⁴²Si and ⁶⁴Cr, for example—the textbook shell model predicts spherical shapes due to the respective magic neutron numbers of 20, 28 and 40 of these nuclei. However, nuclei in these regions turn out to be deformed in their ground states. Another hallmark of these islands is shape coexistence, where a nucleus assumes different shapes with excitation energy. Here we present evidence for this phenomenon from the observation of an excited 0⁺ state in ⁶²Cr, two neutrons away from the heart of the island of inversion around neutron number N = 40. We use large-scale shell-model calculations to interpret the results, and we report extrapolations for the doubly magic nucleus ⁶⁰Ca.

近几十年来，稀有同位素设施使研究短寿命、富含中子的原子核成为可能。它们的测量特性表明，与稳定原子核相比，壳层结构在不平衡的中子-质子比状态下发生了变化。在核图中所谓的倒置岛中，例如，围绕着 ³²Mg、⁴²Si 和 ⁶⁴Cr 的富中子核，教科书式的壳模型由于这些原子核中各自的神奇中子数为 20、28 和 40，从而预测了球形。然而，这些区域的原子核在其基态下会变形。这些岛的另一个特点是形状共存，其中原子核在激发能量下呈现出不同的形状。在这里，我们通过观察 ⁶²Cr 中的激发 0⁺ 状态来提供这种现象的证据，距离中子数 N = 40 周围的反转岛中心有两个中子。我们使用大规模的壳模型计算来解释结果，并报告了双魔核 ^{60 Ca}的外推。