An application of the interacting boson model to the first half of the s-d shell. Pt. 2

A version of the IBM (IBM-4) which is intended for use in regions where protons and neutrons are filling the same shells, and incorporates a boson intrinsic spin, is applied in detail to the odd-odd nuclei 22Na and 26Al. Satisfactory agreement with the observed energies, moments, and transitions is obtained on the introduction of a strong non-central interaction. The eigenfunctions are well described by an enlarged SU(3) symmetry, corresponding to the coupling of orbital and spin factors, in terms of which the spectra of other sd-shell odd-odd nuclei can also be understood. The observed K-spectra are reproduced most naturally on assuming the conventional value of the boson number N. It is argued that the non-central interaction corresponds to a considerable breaking of the ''zeroth-order approximation'' in the mapping of the shell-model spin-orbit force. (orig.)