On the importance of np-pairs in the isovector pairing model

Feng Pan, Liaoning Normal University
Chong Qi, The Royal Institute of Technology (KTH)
Lianrong Dai, Liaoning Normal University
Grigor Sargsyan, Louisiana State University
Kristina D. Launey, Louisiana State University
Jerry P. Draayer, Louisiana State University


It is shown that the isovector np-pair number operator can be realized in the O(5) quasi-spin SUΛ(2) ⊗ SUI(2) basis. The computation of the isovector np-pair number is demonstrated for even-even and odd-odd ds-shell nuclei described by the charge-independent mean field plus isovector pairing model restricted within the O(5) seniority-zero subspace, thereby binding energies and low-lying J = 0+ excited states of these ds-shell nuclei are fit, along with estimates for the isovector neutron-proton pairing contributions. For reasonable neutron-proton pairing strengths the isovector np-pairing energy contribution to the total binding energy in odd-odd N = Z nuclei is systematically larger than that in the even-even nuclei. In sum, the results suggest that the isovector np-pairing mode is favored in odd-odd N = Z nuclei; and additionally, a decrease in the double binding-energy difference for odd-odd nuclei is primarily due to the symmetry and Wigner energy contributions to the binding energy.