The tin isotope ¹⁰⁰Sn is of singular interest for nuclear structure due to its closed-shell proton and neutron configurations. It is also the heaviest nucleus comprising protons and neutrons in equal numbers—a feature that enhances the contribution of the short-range proton–neutron pairing interaction and strongly influences its decay via the weak interaction. Decay studies in the region of ¹⁰⁰Sn have attempted to prove its doubly magic character¹ but few have studied it from an ab initio theoretical perspective^2,3, and none of these has addressed the odd-proton neighbours, which are inherently more difficult to describe but crucial for a complete test of nuclear forces. Here we present direct mass measurements of the exotic odd-proton nuclide ¹⁰⁰In, the beta-decay daughter of ¹⁰⁰Sn, and of ⁹⁹In, with one proton less than ¹⁰⁰Sn. We use advanced mass spectrometry techniques to measure ⁹⁹In, which is produced at a rate of only a few ions per second, and to resolve the ground and isomeric states in ¹⁰¹In. The experimental results are compared with ab initio many-body calculations. The 100-fold improvement in precision of the ¹⁰⁰In mass value highlights a discrepancy in the atomic-mass values of ¹⁰⁰Sn deduced from recent beta-decay results^4,5.

锡同位素¹⁰⁰ Sn 由于其闭壳质子和中子配置而对核结构具有独特的意义。它也是由数量相等的质子和中子组成的最重原子核——这一特征增强了短程质子-中子配对相互作用的贡献，并通过弱相互作用强烈影响其衰变。¹⁰⁰ Sn区域的衰变研究试图证明其双重神奇特性¹但很少有人从从头算理论角度对其进行研究^2,3，并且这些都没有解决奇数质子邻居，它们本质上更难以描述，但对于核力量的完整试验至关重要。在这里，我们展示了奇异奇质子核素¹⁰⁰ In、¹⁰⁰ Sn 和⁹⁹ In的 β 衰变女儿的直接质量测量，其中一个质子小于¹⁰⁰ Sn。我们使用先进的质谱技术来测量每秒仅产生几个离子的⁹⁹ In，并解析¹⁰¹ In 中的基态和异构态。实验结果与 ab initio 多体计算进行了比较。100 的精度提高了¹⁰⁰倍质量值突出了从最近的 β 衰变结果^{4,5推导出的100} Sn的原子质量值的差异。