Investigation of the Δn = 0 selection rule in Gamow-Teller transitions: The β-decay of 207Hg

Gamow-Teller β decay is forbidden if the number of nodes in the radial wave functions of the initial and final states is different. This Δ n = 0 requirement plays a major role in the β decay of heavy neutron-rich nuclei, affecting the nucleosynthesis through the increased half-lives of nuclei on the astrophysical r-process pathway below both Z = 50 (for N > 82 ) and Z = 82 (for N > 126 ). The level of forbiddenness of the Δ n = 1 ν 1 g 9 / 2 → π 0 g7/2 transition has been investigated from the β − decay of the ground state of 207Hg into the single-proton-hole nucleus 207Tl in an experiment at the ISOLDE Decay Station. From statistical observational limits on possible γ-ray transitions depopulating the π0g 7 / 2 − 1 state in 207Tl, an upper limit of 3.9 × 10 − 3 % was obtained for the probability of this decay, corresponding to log ⁡ f t > 8.8 within a 95% confidence limit. This is the most stringent test of the Δ n = 0 selection rule to date.