Spin-aligned neutron-proton pairs in N = Z nuclei

A study is carried out on the role of the aligned neutron-proton pair with angular momentum J = 9 and isospin T = 0 in the low-energy spectroscopy of the N = Z nuclei Cd-96, Ag-94, and Pd-92. Shell-model wave functions resulting from realistic interactions are analyzed in terms of a variety of two-nucleon pairs corresponding to different choices of their coupled angular momentum J and isospin T. The analysis is performed exactly for four holes (Cd-96) and carried further for six and eight holes (Ag-94 and Pd-92) by means of a mapping to an appropriate version of the interacting boson model. The study allows the identification of the strengths and deficiencies of the aligned-pair approximation.