Competition between Allowed and First-Forbidden beta Decay: The Case of Hg-208 -> Tl-2(0)8

The beta decay of Hg-208 into the one-proton hole, one neutron-particle Tl-208(81)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 Q(beta) window, only three negative parity states are populated directly in the beta decay. The data provide a unique test of the competition between allowed Gamow-Teller and Fermi, and first-forbidden beta 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(-)beta decay where the daughter state is core excited is unique, and can provide information on mesonic corrections of effective operators.