Charge radii of neutron-deficient nickel isotopes*

The ⁵⁶Ni nucleus is doubly magic with the neutron number N = 28, which occurs due to a shell gap driven by the spin-orbit force, but is considered to be a soft core. The mean-square charge radii <r²> were determined for ^54,55,56Ni using collinear laser spectroscopy technique at BECOLA at NSCL/MSU to investigate how the softness manifests itself in <r²> at and below N = 28. Details of the experiment and results will be discussed.