The nuclear shell structure, which originates in the nearly independent motion of nucleons in an average potential, provides an important guide for our understanding of nuclear structure and the underlying nuclear forces. Its most remarkable fingerprint is the existence of the so-called magic numbers of protons and neutrons associated with extra stability. Although the introduction of a phenomenological spin–orbit (SO) coupling force in 1949 helped in explaining the magic numbers, its origins are still open questions. Here, we present experimental evidence for the smallest SO-originated magic number (subshell closure) at the proton number six in 13–20C obtained from systematic analysis of point-proton distribution radii, electromagnetic transition rates and atomic masses of light nuclei. Performing ab initio calculations on 14,15C, we show that the observed proton distribution radii and subshell closure can be explained by the state-of-the-art nuclear theory with chiral nucleon–nucleon and three-nucleon forces, which are rooted in the quantum chromodynamics.

A new 11 Be(d, p) 12 Be transfer reaction experiment was carried out in inverse kinematics at 26.9A MeV, with special efforts devoted to the determination of the deuteron target thickness and of the required optical potentials from the present elastic scattering data. In addition a direct measurement of the cross section for the 0 + 2 state was realized by applying an isomer-tagging technique. The s-wave spectroscopic factors of 0.20 +0.03 −0.04 and 0.41 +0.11 −0.11 were extracted for the 0 + 1 and 0 + 2 states, respectively, in 12 Be. Using the ratio of these spectroscopic factors, together with the previously reported results for the p-wave components, the single-particle component intensities in the bound 0 + states of 12 Be were deduced, allowing a direct comparison with the theoretical predictions. It is evidenced that the ground-state configuration of 12 Be is dominated by the d-wave intruder, exhibiting a dramatic evolution of the intruding mechanism from 11 Be to 12 Be, with a persistence of the N = 8 magic number broken.

The elastic scattering and breakup of the halo nucleus 11 Be on protons at an incident energy of 26.9A MeV have been measured. The 11 Be +p elastic scattering cross sections at various energies, including the present one, are systematically analyzed with the Chapel Hill 89 (CH89) and Koning-Delaroche (KD) global optical model potentials (OMPs), and the corresponding normalization factors are obtained. An extended version of the continuum-discretized coupled-channels (XCDCC) formalism, including dynamic core excitation effects, is applied to analyze the elastic scattering and breakup data. It is found that the core excitation plays a moderate role in the elastic scattering and breakup reaction of the halo nucleus 11 Be, being consistent with previous results at higher energies.

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