PROTON-NEUTRON DEGREE-OF-FREEDOM IN NUCLEAR SHAPE

The proton-neutron degree of freedom in nuclear quadrupole shape is discussed in terms of the Proton-Neutron Interacting Boson Model with particular emphasis on M1 transitions and mixed-symmetry states. Mixed-symmetry 2(+) states are searched in Fe-56 and Ba-134. The realistic shell model is used for Fe-56. M1 transitions from the quasi-gamma to quasi-g bands in gamma-unstable or O(6) nuclei are discussed 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 i.e., mixture of mixed-symmetry states. A new mirror-type symmetry is introduced for O(6) nuclei, and selection rules due to this symmetry is presented, making 4(2)(+)-->4(1)(+) transition allowed but 3(1)(+)-->2(1)(+) forbidden, for instance. Results of recent relevant microscopic calculations of the boson Hamiltonian are presented for Xe isotopes.