Does a neutron-proton pairing energy exist?

Abstract

We discuss the mass-data-based expression of a neutron-proton pairing energy presented by Gambhiret al. in their attempt to discover a new odd-even effect which could be an argument for the existence of an alpha-superfluidity of the nucleus, and we compare this expression with another one, derived from the usual definition of the n-n or p-p pairing energy. This discussion shows that it is necessary to distinguish clearly the pairing energies from other interaction energies of valence nucleons: even the usual definition of the neutron or proton pairing energy has to be corrected for the contribution of the neutron-proton interaction energy. A careful analysis of the neutron-proton interaction energies calculated on the basis of experimental mass data, and of their variation as a function of the rank of the partners, could constitute a new approach to the problem of the existence of a neutron-proton pairing energy. Indeed, a study of the n-p interaction energies in theπ, ν 1f7/2 valence shells seems to indicate that true n-p interaction energy and true neutron-proton pairing energy could have different properties. However, the situation in other valence shells remains to be investigated.