Charge Radius of Neutron-Deficient Ni-54 and Symmetry Energy Constraints Using the Difference in Mirror Pair Charge Radii

The nuclear root-mean-square charge radius of Ni-54 was determined with collinear laser spectroscopy to be R(Ni-54) =3.737 (3) fm. In conjunction with the known radius of the mirror nucleus Fe-54, the difference of the charge radii was extracted as Delta R-ch = 0.049 (4) fm. Based on the correlation between Delta R-ch and the slope of the symmetry energy at nuclear saturation density (L), we deduced 21 = L = 88 MeV. The present result is consistent with the L from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within 1 sigma error bands. Our result indicates the neutron-skin thickness of Ca-48 as 0.15-0.21 fm.