The half-life of ${\mathrm{F}}^{20}$ has been determined as 11.03±0.06 sec and $\beta$ branches have been measured in the decay of ${\mathrm{Na}}^{25}$. By combining with information on ${\mathrm{Na}}^{20}$ and ${\mathrm{Si}}^{25}$ it is deduced that for the $A=20\mathrm{}$ mirror decays to the ${\mathrm{Ne}}^{20}$ $2^{+}$ first-excited state, $\frac{{\mathrm{}\left(\mathrm{ft}\right)\mathrm{}}^{+}}{{\mathrm{}\left(\mathrm{ft}\right)\mathrm{}}^{-}}=0.933\mathrm{}\pm 0.032 or 1.062\pm 0.037$, while in the $A=25\mathrm{}$ system $\frac{{\mathrm{}\left(\mathrm{ft}\right)\mathrm{}}^{+}}{{\mathrm{}\left(\mathrm{ft}\right)\mathrm{}}^{-}}=1.187\mathrm{}\pm 0.076$ for $\beta$ decay to all states below 3 MeV.

• Received 16 February 1970

DOI:https://doi.org/10.1103/PhysRevLett.24.1134