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

Abstract

The \ensuremath{\gamma}- or quasi-\ensuremath{\gamma}-band energy staggering in low-spin, low-energy spectra of even-even nuclei is discussed. The staggering indices of \ensuremath{\gamma}-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 \ensuremath{\beta} vibration, respectively. It is found that these staggering indices have opposite signs and provide clear distinctions between \ensuremath{\gamma}-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 \ensuremath{\gamma} dependence. The results show that most nuclei with clear staggering are \ensuremath{\gamma} soft or \ensuremath{\gamma} unstable, but a few may be slightly triaxial, and that almost all the axial nuclei are slightly \ensuremath{\gamma} soft, some of them exhibiting shape transitions from axial to \ensuremath{\gamma}-soft to triaxial shape with increasing angular momentum.