Pairing and Isospin Symmetry in Proton-Rich Nuclei

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Abstract
Unlike their counterparts in the sd shell, most odd-odd N=Z nuclei with A > 40 have ground states with isospin T=1, suggesting an increased role for the isovector pairing interaction. In a simple seniority-like model of this interaction, the effects of like-particle and neutron-proton pairing vary strikingly with the nuclear mass and isospin near N=Z. The variations, which seem to completely elude traditional BCS-like approximations, reflect an interplay between the two kinds of pairs, with the importance of neutron-proton pairing decreasing rapidly as isospin increases and fluctuating in N=Z ground states as the mass increases. Realistic calculations in fp shell nuclei yield very similar patterns, even though deformation and other kinds of correlations are also predicted. The persistence of the patterns in the face of other effects suggests that real proton-rich nuclei exhibit the same competition between collective like-particle and proton-neutron pairs.

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