Persistence of the Z=28 Shell Gap Around Ni78 : First Spectroscopy of

L. Olivier¹,

S. Franchoo²,

M. Nııkura³

et al.

Abstract

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

Skyrme density functional description of the double magic Ni78 nucleus

“…We have found that the tensor term is crucial in obtaining the inversion of the 1f 5/2 and 2p3/2 proton levels around N = 48. This supports the doubly magic character of 78 Ni as observed in recent experiments [3,4] and the conclusion of Ref. [3] that 79 Cu can be described as a 78 Ni core plus a valence proton.…”

“…Such a weakening has been noticed in Ref. [3] in relation to the experimental analysis of the 79 Cu spectroscopy. with an A3DA Hamiltonian [26] performed in Ref.…”

“…of Ref [3]. for the proposed level scheme) lies at 656 keV above the ground state while in our case it lies at 470 keV when tensor is included.…”

Interplay between nuclear shell evolution and shape deformation revealed by the magnetic moment of 75Cu

“…(See "Methods" for related uncertainties). The present structural evolution scenario yields an intriguing prediction of the closed-shell properties of the doubly magic nucleus 78 Ni and its neighbour 79 Cu [26,27]. Their T-plots, shown in Fig.…”

Prominence of Pairing in Inclusive (p,2p) and (p,pnPaul1, Obertelli2, Bertulani3 et al. 2019Phys. Rev. Lett.Self Cite7030View full textAdd to dashboardCite

“…The daughter nuclei were identified via the TOF-Bρ-∆E method in ZeroDegree, operated in large acceptance achromatic mode with a momentum acceptance of ±3%. Details about the experimental campaigns can be found in [49][50][51][52][53][54][55].…”