Recent Coulomb shift calculations for the level structure of the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ nuclei predict that the $3_3^{+}$ state at 2.966 MeV in ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ is the mirror of the 2.646 MeV state in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$, which is of considerable importance for the reaction rate of ${}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}$(p,γ${)}^{20}$Na. Shell model predictions suggest a lifetime of 3.5 fs for the state in ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$, in disagreement with the experimentally determined value of 60±40 fs. The lifetime of low excited states in ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ has therefore been remeasured using the Doppler shift attenuation method (DSAM) and an upper limit of ≤12 fs for the 2.966 MeV state has been obtained.

DOI:https://doi.org/10.1103/PhysRevC.50.R1270