Lifetimes of the First and Second Excited States ofF21

Lifetimes of the 0.279-MeV and 1.10-MeV states produced in the reaction ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}({}_{}{}^7{}_{}{}^{}\mathrm{Li}, \alpha ){}_{}{}^{21}{}_{}{}^{}\mathrm{F}$ were measured using the delayed-coincidence technique. A solid-state detector was used to measure the energies of the alpha particles as well as to produce timing signals. The half-life of the 0.279-MeV state was determined to be (7.03±0.50)×${10}^{-9\mathrm{}}$ sec and that of the 1.10-MeV state less than 2×${10}^{-10\mathrm{}}$ sec. The lifetime of the 0.279-MeV level implies an $E2\mathrm{}$ transition and ${\mathrm{½}}^{+}$ assignment for the spin. This result is in agreement with two-nucleon stripping fits. The level order can be understood on the basis of the rotational model with band mixing using a lower distortion for ${}_{}{}^{21}{}_{}{}^{}\mathrm{F}$ than for neighboring $T=\frac{1}{2}$ nuclei. The measured lifetimes are in qualitative agreement with this model.