Staggering in γ-band energies and the transition between different structural symmetries in nuclei

The experimental energy staggering in γ bands of rare earths and actinides exhibits three distinct patterns as a function of angular momentum that are typical of well-deformed structural benchmarks: γ-soft for nuclei situated between a vibrator and a deformed γ-soft structure, axially symmetric for those between a vibrator and a rigid rotor, and triaxial γ-rigid for nuclei between a vibrator and a rigid triaxial rotor. The three patterns are reproduced by appropriate special solutions of the Bohr Hamiltonian, as well as by interacting boson approximation calculations. A particular quantity called S(4), which is proportional to the displacement of the 3_γ⁺ level relative to the average of the 2_γ⁺ and 4_γ⁺ levels, can vary in magnitude and sign for different shapes and is found to give a good indication of structure and the evolution of structure. A sudden change in the γ-band staggering occurring between the vibrator and the axially symmetric rotor limits seems to be connected to the known presence of a first-order phase/shape transition in this region.