Characterisation of the few doubly magic nuclei, known and predicted, provides a benchmark for our knowledge of the fundamental forces that drive the evolution of shell closures with proton-to-neutron asymme
In-beam γ-ray spectroscopy of 79 Cu is performed at the Radioactive Isotope Beam Factory of RIKEN. The nucleus of interest is produced through proton knockout from a 80Zn beam at 270 MeV=nucleon. The level scheme up to 4.6 MeV is established for the first time and the results are compared to Monte Carlo shell-model calculations. We do not observe significant knockout feeding to the excited states below 2.2 MeV, which indicates that the Z ¼ 28 gap at N ¼ 50 remains large. The results show that the 79 Cu nucleus can be described in terms of a valence proton outside a 78Ni core, implying the magic character of the latter. DOI: 10.1103/PhysRevLett.119.192501 The shell model constitutes one of the main building blocks of our understanding of nuclear structure. Its robustness is well proven for nuclei close to the valley of stability, where it successfully predicts and explains the occurrence of magic numbers [1,2]. However, these magic numbers are not universal throughout the nuclear chart and their evolution far from stability, observed experimentally over the last decades, has generated much interest [3]. For example, the magic numbers N ¼ 20 and 28 may disappear [4][5][6][7] while new magic numbers arise at N ¼ 14, 16 and 32, 34, respectively [8][9][10][11][12][13]. Although shell gaps, defined within a given theoretical framework as differences of effective single-particle energies (ESPE), are not observables [14], they are useful quantities to assess the underlying structure of nuclei [15][16][17]. The nuclear potential acting on nuclei far from stability can induce drifts of the single-particle orbitals and their behavior as a function of isospin can be understood within the shell model [18][19][20][21][22]. Difficulties arise, however, when the single-particle properties are masked by correlations that stem from residual interactions and discriminating between the two effects is nontrivial.In the shell model as it was initially formulated, the proton πf 7=2 orbital separates from the 3ℏω harmonic oscillator shell because of the spin-orbit splitting and forms the Z ¼ 28 gap. The neutron νg 9=2 orbital splits off from the 4ℏω shell to join the 3ℏω orbits and creates a magic number at N ¼ 50. With 28 protons and 50 neutrons, the 78 Ni nucleus is thus expected to be one of the most neutronrich doubly magic nuclei, making it of great interest for nuclear structure. Up to now, no evidence has been found for the disappearance of the shell closures at Z ¼ 28
One of the most exotic light neutron-rich nuclei currently accessible for experimental study is 40 Mg, which lies at the intersection of the nucleon magic number N =28 and the neutron drip-line. Low-lying excited states of 40 Mg have been studied for the first time following a one-proton removal reaction from 41 Al, performed at RIBF RIKEN with the DALI2 γ-ray array and the Zero-Degree spectrometer. Two γ-ray transitions were observed, suggesting an excitation spectrum which shows unexpected properties compared to both the systematics along the Z=12, N ≥20 Mg isotopes and available state-of-the-art theoretical model predictions. A possible explanation for the observed structure involves weak-binding effects in the low-lying excitation spectrum.
We report the measurement of reaction cross sections (σ ex R ) of 27,29 F with a carbon target at RIKEN. The unexpectedly large σ ex R and derived matter radius identify 29 F as the heaviest twoneutron Borromean halo to date. The halo is attributed to neutrons occupying the 2p 3/2 orbital, thereby vanishing the shell closure associated with the neutron number N = 20. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of 27 F but are challenged for 29 F.
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