Measurement of the $2^{+}\to 0^{+}$ ground-state transition in the ${\hat{I}}^2$ decay of ${}^{20}$F

Abstract

We report the first detection of the second-forbidden, non-unique, $2^{+}\to 0^{+}$, ground-state transition in the ${\hat{I}}^2$ decay of ${}^{20}$F. A low-energy, mass-separated ${}^{20}{\mathrm{F}}^{+}$ beam produced at the IGISOL facility in Jyväskylä, Finland, was implanted in a thin carbon foil and the ${\hat{I}}^2$ spectrum measured using a magnetic transporter and a plastic-scintillator detector. The ${\hat{I}}^2$-decay branching ratio inferred from the measurement is $b_{\hat{I}}^2=[0.41\pm 0.08\text{(stat)}\pm 0.07\text{(sys)}]\times 10^{-5}$ corresponding to $\log ft=10.89(11)$, making this one of the strongest second-forbidden, non-unique ${\hat{I}}^2$ 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.

13 pages, 16 figures, 4 tables