We used our (perviously described) system for high sensitivity measurements of β-delayed α-particle emission of light nuclei to measure upper limits of the branching ratios of the first-forbidden rank-one β decay of the $2^{+}$ ground state of ${}^{20}F$ to the $3^{-},5.62$ MeV, and $1^{-},5.79$ MeV excited states of ${}^{20}\mathrm{Ne}$ to be $1.5\times {10}^{-6}$ and $7\times {10}^{-7}$, respectively. These limits are more than a factor of 300 smaller than previously measured. The obtained $\mathit{ft}$ values are at least a factor of 10 smaller than that of the first-forbidden rank-zero β decay to the $2^{-}$ state at $4.97$ MeV in ${}^{20}\mathrm{Ne}$, whose branching ratio was also measured to be $1.2(6)\times {10}^{-4}$, in agreement with the precise value obtained by Alburger and Warburton of $0.90(4)\times {10}^{-4}$.

• Received 3 April 2007

DOI:https://doi.org/10.1103/PhysRevC.76.018501