The decay of mass-separated ¹⁹³TL : interpretations based on a particle-core coupling model

The β⁺/EC-decay of ¹⁹³Tl to ¹⁹³Hg was investigated using mass-separated sources produced at the UNISOR facility, operating on-line to the Holifield Heavy-Ion Research Facility's Tandem Accelerator at Oak Ridge National Laboratory. The reaction of ¹⁶O on a stacked-foil Re/Mo target was used to produce ¹⁹³Bi, which β⁺-decayed to ¹⁹³Pb which in turn β⁺-decayed to ¹⁹³Tl. Multiscaled γ-singles and electron-singles, as well as γ-γ and electron-γ coincidence data were taken. Analysis of the data has yielded an extensive decay scheme of 191 γ rays between 88 levels of both positive and negative parity. 31 transitions and 23 levels are new or confirmed as compared to the most recent study (J. L. Wood, private communication). Energies and intensities have been measured for both γ rays and conversion electrons. Spin and parity assignments of the levels were made based on γ-ray multipolarities derived from measured conversion coefficients. Evidence for shape-coexistence in ¹⁹³Hg was discovered in five transitions with E0 components. One of these transitions was found to decay from an initial level of 1580.7 keV. This is close to the expected energy for deformed states according to systematics of states with prolate deformation intruding into the slightly oblate states of Hg nuclei in the region 182 ≤ A ≤ 190 (D. Proetel et al., 1973; N. Rud et al., 1973; J. H. Hamilton et al., 1975; J. D. Cole et al., 1977; J. D. Cole et al., 1976; J. H. Hamilton et al., 1985; M.O. Kortelahti et al., 1989; J. Bonn et al., 1976). A particle-core coupling model has been used to predict and interpret energy levels and their properties for low-lying levels in ¹⁹³Hg; relative intensities for selected transitions have also been calculated. In the version of the model used here, the mean field is parameterised with a deformed modified oscillator potential, and the core energy spectrum is described as a rigid triaxial rotor. Virtually all the model parameters were fixed at their standard values. Comparison between these calculations and the data provides strong evidence for ground state triaxial deformation in ¹⁹³Hg.