Systematics of β- and γ-bands in the A = 160 region

Full text: By considering the nucleus as a vibrating liquid drop, and assuming the potential to be a function of the elongation β, and triaxiality γ, of the nucleus, the Bohr Hamiltonian can be solved to give the so-called K=0⁺, γ-vibrational and K=2⁺ γ-vibrational bands. However, as summarized in the review by Garrett, very few of the observed 0⁺₂ bands in deformed nuclei possess the properties expected of a β vibration. It is likely that the composition of the wavefunctions of the 0⁺₂ states vary with the Fermi level, and contain admixtures of β-vibrational, two-phonon, pairing, and shape-coexisting states. At iThemba LABS, a systematic investigation of low-lying levels in the mass 160 region is underway. An extensive set of data on the low-lying, positive-parity bands in the nuclides between N = 88 and 92 and Sm to Yb has been obtained from γ-γcoincidence measurements following fusion-evaporation reactions optimized of the population of low-spin states. Some these results point to the role of quadrupole pairing in forming 0⁺₂ bands. In this work, the energies and electromagnetic properties of the so-called β- and γ-bands of nuclei in this region are compared with the solutions of a five dimensional collective Hamiltonian for quadrupole vibrational and rotational degrees of freedom, with moments-of-inertia and mass parameters determined by constrained self-consistent relativistic mean-field calculations using the PC-F1 relativistic functional. A good qualitative agreement is found between the measured energies and of the in-band/out-of-band branching ratios across the entire region. (author)