Charge Radius of Neutron-deficient $^{54}$Ni and Symmetry Energy Constraints Using the Difference in Mirror Pair Charge Radii

Abstract

The nuclear root-mean-square charge radius of 54 Ni was determined with collinear laser spectroscopy to be R( 54 Ni) = 3.737 (3) fm. In conjunction with the known radius of the mirror nucleus 54 Fe, the difference of the charge radii was extracted as ∆R ch = 0.049 (4) fm. Based on the correlation between ∆R ch and the slope of the symmetry energy at nuclear saturation density (L), we deduced 20 ≤ L ≤ 70 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 1σ error bands. Our result indicates the neutron-skin thickness of 48 Ca as 0.15 -0.19 fm.