Staggering in low-spin nuclear spectra of γ-soft or triaxial nuclei

The γ- or quasi-γ-band energy staggering in low-spin, low-energy spectra of even-even nuclei is discussed. The staggering indices of γ-soft and triaxial nuclei are calculated in the interacting boson model O(6) limit with three-body potential and general axially asymmetric rotor model with β vibration, respectively. It is found that these staggering indices have opposite signs and provide clear distinctions between γ-soft and triaxial shapes. The experimental data of energy spectra of 140 of even-even nuclei in the mass region A=64 to 200 are compared to the predictions for broken O(6) symmetry with varying strength of the three-body interaction, and for the axially asymmetric rotor model with varying γ dependence. The results show that most nuclei with clear staggering are γ soft or γ unstable, but a few may be slightly triaxial, and that almost all the axial nuclei are slightly γ soft, some of them exhibiting shape transitions from axial to γ-soft to triaxial shape with increasing angular momentum