Competition between allowed and first forbidden β decays of 208 At and expansion of the 208 Po level scheme

The structure of 208 Po populated through the EC/β + decay of 208 At was investigated using γ-ray spectroscopy at the ISOLDE Decay Station. The presented level scheme contains 27 new excited states and 43 new transitions, as well as a further 50 previously-observed γ rays which have been (re)assigned a position. The level scheme was compared to shell model calculations. Through this analysis approximately half of the β decay strength of 208 At was found to proceed via allowed and half via first forbidden decay. The first forbidden transitions predominantly populate core excited states at high excitation energies, which was qualitatively understood using shell model considerations. This mass region provides an excellent testing ground for the competition between allowed and first forbidden β-decay calculations, important for the detailed understanding of the nucleosynthesis of heavy elements