  We report the first detection of the second-forbidden, non-unique,
$2^+\rightarrow 0^+$, ground-state transition in the $\beta$ decay of $^{20}$F.
A low-energy, mass-separated $^{20}\rm{F}^+$ beam produced at the IGISOL
facility in Jyv\"askyl\"a, Finland, was implanted in a thin carbon foil and the
$\beta$ spectrum measured using a magnetic transporter and a
plastic-scintillator detector. The $\beta$-decay branching ratio inferred from
the measurement is $b_{\beta} = [ 0.41\pm 0.08\textrm{(stat)}\pm
0.07\textrm{(sys)}] \times 10^{-5}$ corresponding to $\log ft = 10.89(11)$,
making this one of the strongest second-forbidden, non-unique $\beta$
transitions ever measured. The experimental result is supported by shell-model
calculations and has significant implications for the final evolution of stars
that develop degenerate oxygen-neon cores. Using the new experimental data, we
argue that the astrophysical electron-capture rate on $^{20}$Ne is now known to
within better than 25% at the relevant temperatures and densities.

Nuclear Experiment