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

Neutron single-particle strengths at N=40 , 42: Neutron knockout from Ni68,70 ground and isomeric states

“…The first excited state of 68 Ni, a 0 + 2 level at 1604 keV [1,11], hints at the possibility of shape coexistence at low excitation energy. Other evidence comes from the existence of a low-lying 0 + 3 state and from the observed decay patterns of the associated 2 + states [11,12]. All of these experimental observations, in agreement with theoretical predictions by Tsunoda et al [2], point to a rather peculiar structure for this nucleus where spherical, oblate, and prolate shaped structures coexist in a narrow energy range, differing in binding energy by at most 2.51 MeV.…”

“…Also, SM calculations do not predict such a scenario that would lead to isomers beyond those already observed for 68 Ni. Furthermore, the existing body of data for 68 Ni is extensive and provides no indication of isomers beyond the ones observed thus far [1,11,12,21,22].…”

“…This leads to deformation that may enhance specific configurations, for instance, those that are characterized by a higher neutron occupancy of the g 9/2 and d 5/2 orbitals. Recent relevant information on 67−70 Ni stems from a number of measurements using a wide variety of experimental techniques, such as β decay, deep-inelastic reactions, searches for isomeric states, and transfer reactions [1,11,12,[19][20][21][22][23][24][25].…”

New low-energy0+state and shape coexistence inNi70

“…Three 0 + states in 68 28 Ni 40 , at 0, 1604 [15,16], and 2511 keV [17] associated with multiple particle-hole excitations across Z = 28 and N = 40 [18,19], have been interpreted in terms of spherical, oblate, and prolate shapes based on comparisons with Monte Carlo ShellModel (MCSM) calculations [15,20]. The presence of spherical-prolate shape coexistence in the lighter 66 Ni [21], as well as in 68 Ni [21,22], is also expected based on mean-field calculations.…”

In-medium similarity renormalization group for closed and open-shell nuclei

“…A possible reason could be the enforcement of spherical symmetry in our calculations. Recent experiments have revealed the coexistence of spherical ground states and axially deformed states with excitation energies below 3 MeV in 68 Ni and its vicinity [183][184][185]. In the next subsection, we will discuss examples in which the MR-IMSRG(2) successfully deals with the presence of both spherical and deformed states in the spectrum of neon isotopes, but we note that the states in question have much larger energetic separations of 7 − 8 MeV.…”