Competition between Allowed and First-Forbidden $β$ Decay: The Case of $^{208}$Hg$→^{208}$Tl

The $β$ decay of $^{208}$Hg into the one-proton hole, one neutron-particle $^{208}_{81}$Tl$_{127}$ nucleus was investigated at CERN-ISOLDE. Shell-model calculations describe well the level scheme deduced, validating the proton-neutron interactions used, with implications for the whole of the N>126, Z<82 quadrant of neutron-rich nuclei. While both negative and positive parity states with spin 0 and 1 are expected within the Qβ window, only three negative parity states are populated directly in the β decay. The data provide a unique test of the competition between allowed Gamow-Teller and Fermi, and first-forbidden $β$ decays, essential for the understanding of the nucleosynthesis of heavy nuclei in the rapid neutron capture process. Furthermore, the observation of the parity changing $0^+→0^-β$ decay where the daughter state is core excited is unique, and can provide information on mesonic corrections of effective operators.