Magnetic dipole transitions and proton-neutron degrees of freedom in nuclear deformation

Abstract

The M1 transitions between low-lying collective states are discussed from the viewpoint of the Proton-Neutron Interacting Boson Model, with particular emphasis on the mixed-symmetry states. Mixed-symmetry 2+ states are studied for56Fe and54Cr in terms of realistic and large-scale shell-model calculations, including M1 properties. The Doorway-state character of the mixed-symmetry 2+ state is proposed with examples in these nuclei. The possible candidate of the mixed-symmetry 2+ state in134Ba is analyzed based on recent experiment by Molnaret al. on M1 transitions. The M1 transitions from the quasi-γ to quasi-g bands in γ-unstable or O(6) nuclei are discussed next, by taking Ba isotopes as an example. It is suggested that such M1 transitions are enhanced compared to axially symmetric nuclei as an indication of softness towards proton-neutron incoherent motion in γ-unstable nuclei,i.e., mixture of mixed-symmetry states. A new mirror-type symmetry is introduced for γ-unstable nuclei, and the M1 selection rule due to this symmetry is presented, making 4 2 + →4 1 + transition allowed but 3 1 + →2 1 + forbidden, for instance.