“Stiff” deformed nuclei, configuration dependent pairing and the $\beta$β and $\gamma$γ degrees of freedom

Abstract

We review the current experimental data on collective structures within the pairing gap of even-even deformed nuclei, with emphasis on nuclei near mass number $A \sim 150$. The essential physics that determines the characteristics of the first excited 0+ (02+) state in these nuclei has been in dispute for several decades. Interpretation of these states in terms of surface $\beta$ quadrupole vibrations has often been challenged. We examine the role that configuration dependent pairing can play in these levels particularly at the onset of deformation as major shells fill. In all deformed nuclei rotational bands are found experimentally, starting at a state with spin 2+ with excitation energies near the middle of the pairing gap. These rotational bands, with quantum number $K^{\pi} = 2^{+}$, are usually referred to as $\gamma$ bands and have been identified with quadrupole surface vibrations in the plane perpendicular to the major axis of deformation. However $K^{\pi} = 2^{+}$ bands can also arise due to the breaking of axial symmetry of the quadrupole shape. We discuss data that can help with these different interpretations.