How magic is the magic68Ninucleus?

Abstract

We calculate the B(E2) strength in 68 Ni and other nickel isotopes using several theoretical approaches. We find that in 68 Ni the gamma transition to the first 2 + state exhausts only a fraction of the total B(E2) strength, which is mainly collected in excited states around 5 MeV. This effect is sensitive to the energy splitting between the f p shell and the g 9/2 orbital. We argue that the small experimental B(E2) value is not strong evidence for the double-magic character of 68 Ni.The appearance of shell gaps associated with magic nucleon numbers is one of the cornerstones of nuclear structure. The presence of magic gaps allows one, for example, to determine the single-particle energies and the residual interaction among valence nucleons, providing essential input for nuclear models. Magic gaps offer a natural way of performing truncations in microscopic many-body calculations. Magic nuclei also play an essential role in the two major nucleosynthesis networks (s-and r-process) that produce the majority of nuclides heavier than mass number A ∼ 60.The doubly-magic character of 68 Ni (Z=28, N =40) was suggested in the early eighties [1,2] and tested experimentally [4][5][6][7]. The proton number Z=28 in the nickel isotopes is magic. In the neutrons, the sizeable energy gap at N =40 separates the pf spherical shell from the g 9/2 intruder orbit. However, this spherical subshell closure is not sufficiently large to stabilize the spherical shape. [Experimentally [3], 80 Zr (N =Z=40) behaves like a well-deformed rotor.] The current experimental evidence about the double-magicity of 68 Ni is controversial [8]. On the one hand, 68 Ni does not show a pronounced irregularity in the two-neutron separation energies, as is expected for a magic nucleus. On the other hand, the lowered position of the 0 [10], the size of the N =40 gap strongly depends on the effective interaction used, and it dramatically influences the quadrupole collectivity of the N =40 nuclei. While there is much discussion in the literature about the weakening of shell effects in neutronrich nuclei (e.g., the magic gap N =28 seems to be eroded in drip-line systems; see Ref.[10] and references quoted therein), 68 Ni lies very far from the expected neutron drip line (expected to be around 92 Ni [11]) and one should probably not invoke "exotic" explanations when discussing the stucture of this neutron rich nucleus.It is the aim of this Letter to draw attention to the total B(E2; 068 Ni, which can hold the key to the question whether this nucleus is magic or not. We will argue that the transition to the first excited 2 + state constitutes only a small part of the total B(E2) strength and that the total B(E2) strength depends sensitively on the size of the N = 40 shell gap.To understand the structural difference between 68 Ni and 56 Ni (where the transition to the first 2 + state exhausts most of the total low-energy strength), we begin from qualitative arguments based on a simple independent particle model (IPM). Proton configurations in both nuclei are id...

Tensor part of the Skyrme energy density functional. II. Deformation properties of magic and semi-magic nuclei

“…Indeed, the odd-parity pf shell is completely filled and the 1g 9/2 + orbital empty [49,50] and at least a 2p-2h excitation is needed to construct a positive-parity state. The interpretation of 68 Ni as a doubly-magic nucleus has also been questioned by shell model and QRPA calculations [51]. Let us mention, finally, that the first excited state of 68 Ni is a 0 + level with a small B(E0) value to the ground state [52], pointing to a possible shape coexistence with weak mixing.…”

Low-spin states and the non-observation of a proposed 2202-keV,0+isomer in68Ni

“…Subsequent work in the region revealed the transitory nature of the latter shell gap, as the magicity was found not to persist in other N ≈ 40 nuclei [3][4][5][6] and some theoretical efforts such as those in Refs. [7][8][9][10] have called into question the N = 40 magicity in 68 Ni itself. The weakening of the N = 40 shell gap, as well as the generation of collectivity in this region, have drawn considerable attention to the influence of neutron excitations across the N = 40 and N = 50 shell gaps into the g 9/2 and d 5/2 singleparticle states, respectively, and of excitations across the Z = 28 gap, e.g.…”

Identification of deformed intruder states in semi-magicNi70

“…The structure of neutron-rich nuclei in the vicinity of 68 Ni has been the subject of considerable scrutiny in recent years, including investigations related to the magicity of 68 Ni [1][2][3][4][5][6][7][8][9][10], the existence of multiple coexisting shapes [11][12][13][14], the presence of intruder states originating from orbitals across a shell gap [15][16][17][18][19][20][21][22], and the unexpected disappearance of seniority isomers [23][24][25][26][27][28][29]. The delineation of levels up to several MeV in excitation energy in these nuclei is an important step towards providing a firmer understanding of their properties through comparisons with modern theoretical models.…”