Signature inversion in odd-odd nuclei in the interacting boson-fermion model

The signature inversion in odd-odd nuclei is studied in terms of the interacting boson-fermion model (IBFM). Energy levels and electromagnetic matrix elements are calculated in the model in which a proton and a neutron are coupled to the SU(3) (deformed) and the O(6) (γ-unstable) IBM2 cores. When the particles occupy high-j orbits, the signature inversion is caused in the O(6) limit by strong exchange interactions between fermions and the O(6) core. The essential mechanism of the inversion is the mixing of the members of the quasi-γ band into the unfavored states due to the exchange interactions. In the case of the SU(3) core, the inversion never occurs when the particles are in high-j orbits. It is seen only around the band head when the fermion orbits have small angular momenta. (orig.)