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

¹ Department of Physics, Liaoning Normal University - Dalian 116029, China
² Department of Physics and Astronomy, Louisiana State University - Baton Rouge, LA 70803-4001, USA
³ Department of Physics, KTH Royal Institute of Technology - 10691 Stockholm, Sweden
⁴ Department of Physics, School of Science, Huzhou University - Huzhou, Zhejiang 313000, China

Received: 4 September 2020
Accepted: 14 October 2020

Abstract

It is shown that the isovector np-pair number operator can be realized in the O(5) quasi-spin $SU_{\Lambda}(2)\otimes SU_{I}(2)$ 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.