Charge radii behavior in Ni and Ca at N=28 surprisingly similar

The results of our investigation on the charge radius of neutron deficient nickel isotopes across the doubly magic nickel-56 appeared today in Physical Review Letters. Measurements were carried out at the BECOLA facility at NSCL/MSU.
We found a surprising similarity between the kink at N=28 in nickel and calcium isotopes, even though it is known that the N=28 shell closure in nickel-56 is much weaker than in calcium-48. Theoretical calculations applying either density functional theory or using variants of ab initio models are able to reproduce this behavior very well. This suggests that the magnitude of the kink does not directly reflect the strength of the shell-closure.
For details see here.