Magicity around 68Ni

The structure of ⁶⁸Ni with a closed proton shell at Z=28 and a subshell closure at N=40 possesses certain nuclear properties that are characteristic for a doubly magic nucleus: a high energy of the first excited 2⁺ state and a low quadrupole transitional matrix element connecting the 0⁺ ground state to this first excited 2⁺ state. Still, mass measurements, the most sensitive and direct signature for (sub)shell closures, don't establish a clear neutron shell gap at N=40 between the pf-shell and the g_9/2 orbital. It seems that at N=40, halfway between N=28 and N=50, a subtle interplay arises between stabilising and mid-shell effects.

A better knowledge of the residual interactions between neutrons in the g_9/2 orbital and protons in the f_7/2 and f_5/2 orbitals is required to get a deeper insight. Since, roughly speaking, the same orbitals interact with each other in ⁷⁸Ni as in ⁶⁸Ni, it will also allow for more accurate extrapolations of shell model predictions towards ⁷⁸Ni.

Therefore, not only ⁶⁸Ni itself is of great importance, but also the nuclei adjacent to it. Our group is active in this region with complementary techniques. At LISOL, the nuclear structure of ^68-74Cu (Z=29), ^65-71Ni and more recently also ^65-67Co (Z=27) has been investigated through beta-decay studies. At ISOLDE, Coulomb excitation experiments have been performed on ⁶⁸Ni and the neighboring odd-A and odd-odd copper isotopes to measure the collectivity of excited levels.

The highlight of the most recent Coulomb excitation experiments are the collective character of the 5/2- state in odd copper isotopes before N=40 and single particle character from N=40 onwards. The discovery of proton intruder states in ^65-67Co are recent beta-decay study highlights. Transfer reactions to study the single particle strengths of levels in ⁶⁷Ni are planned in 2009 at ISOLDE and ongoing developments on the LISOL laser ion source could open perspectives to study the ⁶⁸Co level scheme.