Halo Structure of Isobaric Analog States in A = 21 and 17 Mirror Nuclei

The root-mean-square (rms) radius of the last nucleon in the 2s_1/2 state of ²¹Ne, ²¹Na, ¹⁷O, and ¹⁷F are found to be 4.4±0.5, 5.2±0.6, 4.3±0.5, and 5.0±0.6 fm, respectively, from transfer reaction data. The results show that the 2s_1/2 states of ²¹Na and ¹⁷F are proton halo states, while the analogous states in their mirror nuclei ²¹Ne and ¹⁷O can be considered as neutron skin states. Comparisons among the rms radii of these states lead us to expect that a neutron halo nucleus has a proton halo mirror partner, and the Coulomb barrier is a determinant factor limiting the extension of the rms radius of the loosely bound proton.