Abstract
Data collected during a decay spectroscopy experiment at the ISOLDE Decay Station (IDS) in 2016, were used to investigate the structure of 208Po populated via the β+/EC decay of 208At. This decay, and resulting structure, was most recently studied in experiments which took place in the 1980s. Thus, the aim of this analysis was to take advantage of improvements in detector technology and radioactive beam production to establish an expanded level scheme below QEC = 4999(9) keV. From this work a total of 170 transitions and 58 states were observed, including 27 newly-observed states. Ninety-three of the transitions identified in this analysis differ from prior decay studies. These consist of 43 newly-observed transitions; 33 previously-observed 208Po γ rays placed in the level scheme through this analysis; and 17 transitions moved from previous placements. Spin-parities were assigned through a combination of γ-ray placements, previously-measured αK values, and restrictions resulting from β and γ decay selection rules. In addition, electron conversion coefficients were calculated for all transitions below 1 MeV, and log ft values were determined for all populated states using intensity imbalances. From this analysis, ∼46% of the decays were found to be first-forbidden, a significant increase from the ∼37% measured in the 1980s. It is suggested that the high first-forbidden proportion of 208At and surrounding β+/EC-decaying nuclei could provide a testing ground for models of first-forbidden decay. These can then be incorporated into larger β-decay studies in the region which lack relevant data points, such as models of r-process nucleosynthesis. In addition, a low-lying 3− state at 1995 keV, which had been previously assigned (2−,3−), was identified. Through comparisons with shell model calculations and considering the underlying shell structure, it was concluded that the low energy of the state was the result of configuration mixing of an abundance of 3− states. In particular, the strength of f7/2i13/2 contributions to the octupole from both proton particle and neutron hole excitations results in stronger mixing for 3− states in 208Po with dominant f7/2i13/2 configurations. Strong octupole collectivity in the region, and observed, low-energy, core-excited states in neighbouring nuclei suggest collective character for the 1995 keV state. Although no evidence of this was found in this analysis, it is anticipated that a high statistics Coulex experiment could provide further detail into the nature of the 1995 keV state.