The calcium isotopes are an ideal system to investigate the evolution of shell structure and magic numbers. Although the properties of surface nucleons in calcium have been well studied, probing the structure of deeply bound nucleons remains a challenge. Here, we report on the first measurement of unbound states in Ca and Ca, populated from Ca() reactions at a beam energy of around 216 MeV/nucleon at the RIKEN Radioactive Isotopes Beam Factory. The resonance properties, partial cross sections, and momentum distributions of these unbound states were analyzed. Orbital angular momentum assignments were extracted from momentum distributions based on calculations using the distorted wave impulse approximation (DWIA) reaction model. The resonances at excitation energies of 5516(41)keV in Ca and 6000(250)keV in Ca indicate a significant =3 component, providing the first experimental evidence for the single-particle strength of unbound hole states in the neutron-rich Ca isotopes. The observed excitation energies and cross-sections point towards extremely localized and well separated strength distributions, with some fragmentation for the orbital in Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and calculations and diverge markedly from the experimental distributions in the nickel isotones at .

中文翻译：

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富中子 Ca 同位素深束缚轨道的光谱学


钙同位素是研究壳结构和幻数演化的理想系统。尽管钙表面核子的性质已得到充分研究，但探测深层结合核子的结构仍然是一个挑战。在这里，我们报告了在 RIKEN 放射性同位素束工厂对 Ca 和 Ca 中未结合态的首次测量，这些状态是由 Ca()2 反应在大约 216 MeV/核子的束流能量下填充的。分析了这些非束缚态的共振特性、部分横截面和动量分布。根据使用扭曲波脉冲近似（DWIA）反应模型的计算，从动量分布中提取轨道角动量分配。 Ca 中 5516(41)keV 和 Ca 中 6000(250)keV 激发能下的共振表明存在显着的 =3 分量，为富中子 Ca 同位素中未束缚空穴态的单粒子强度提供了第一个实验证据。观察到的激发能和横截面指向极其局部和分离良好的强度分布，Ca 轨道有一些碎片。这些结果与使用有效 GXPF1Bs 相互作用和计算的壳模型计算的预测非常一致，并且与 处镍同位素的实验分布明显不同。