Shape Coexistence in 68Ni and surrounding neutron-rich nuclei
The structure of neutron-rich nuclei in the vicinity of “doubly-magic” 68Ni has been the subject of much recent scrutiny once it was realized, on the one hand, that the subshell gap at neutron number N=40 might be rather small and, on the other, that particle-hole excitations might result in the presence of states associated with different nuclear shapes. A set of recent papers [1-3], combining data from fragmentation reactions at the NSCL with those from multi-nucleon transfer reactions at ATLAS, has resulted in an interpretation of the low-spin level structure of 66,68,70Ni (see Fig. 1) in terms triple shape coexistence. In Ni, for example, 3 families of levels coexist: one associated with the spherical ground state, another with the oblate-deformed 0+ level at 1604 keV and a third built on a prolate, 2511-keV excitation. The observations agree with the results of Monte Carlo shell-model calculations by the Tokyo group  which attribute the presence of prolate shapes with large deformation to the role of proton particle-hole excitations across the Z=28 shell gap combined with the shape-driving effects of neutrons occupying the g9/2 and d5/2 orbitals. It is worth noting that the prolate excitation decreases drastically in energy with increasing neutron number, an effect that can be traced to the tensor component of the monopole interaction .
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