Shell Evolution towards Ni78 : Low-Lying States in Cu77

Abstract

The level structure of the neutron-rich 77 Cu nucleus is investigated through β-delayed γ-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of 77 Ni are produced by inflight fission, separated and identified in the BigRIPS fragment separator, and implanted in the WAS3ABi silicon detector array, surrounded by Ge cluster detectors of the EURICA array. A large number of excited states in 77 Cu are identified for the first time by correlating γ rays with the β decay of 77 Ni, and a level scheme is constructed by utilizing their coincidence relationships. The good agreement between large-scale Monte Carlo shell model calculations and experimental results allows for the evaluation of the singleparticle structure near 78 Ni and suggests a single-particle nature for both the 5=2 − 1 and 3=2 The evolution of the shell structure is one of the key motivations to study atomic nuclei with large neutron excess. The goal is to understand effects due to this excess of neutrons that are responsible for deviations from the conventional harmonic oscillator description with a strong attractive spin-orbit coupling, which characterizes the shell structure and properties of nuclei near the line of β stability. Such deviations are related to the monopole components of the effective nucleon-nucleon interaction and their strong effects on the single-particle energies (SPEs). The spindependent central component influences the energies of all single-particle orbitals, while the tensor interaction alters the spin-orbit splitting when specific orbits are filled by neutrons or protons [1][2][3][4][5][6][7][8].For the chain of Ni (Z ¼ 28) isotopes between N ¼ 40 and N ¼ 50, theoretical models predict significant changes in the proton SPEs as the ν1g 9=2 shell is filled by neutrons [3,4,[9][10][11][12]. Here, the tensor force responsible for SPE shifts

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

“…In nuclear medium, the isovector correction based on the meson-exchange picture is made usually: g ℓ (p) = g free ℓ (p) + δg ℓ and g ℓ (n) = g free ℓ (n) − δg ℓ [35,36], with the empirical value of δg ℓ being between 0 [38] and 0.1 [37]. The measured magnetic moments of 75 Cu are 1.40 (6)µ N for the first 3/2 − state as obtained by the present work, and 1.0062 (13)µ N for the first 5/2 − state [11]. We can evaluate the values of q s and δg ℓ from them.…”

78Ni revealed as a doubly magic stronghold against nuclear deformation

“…After the first production of 78 Ni 17 , enormous efforts have been put into investigating its structure. Previous measurements indirectly inferred persistent N = 50 [18][19][20][21][22] and Z = 28 [23][24][25] shell closures at 78 Ni. This notion has been reinforced theoretically by ab initio predictions 27 .…”

Excitations of the magic N=50 neutron-core revealed in Ga81

“…The Z = 28 shell closure is well known for exhibiting a strong magic behavior. However, in Cu isotopes (Z = 29), the lowering of the 1 f 5/2 orbital induces a reduction of ≈2 MeV of the Z = 28 shell gap between N = 40 and N = 50 [12]. Further, many efforts have been devoted to the characterization of the N = 50 shell gap [13][14][15][16][17][18][19][20] and support a possible weakening of the gap at Z = 32 [21,22], whereas a persistent N = 50 gap has been suggested at Z = 30 [19].…”