The recoil distance method was used to measure the lifetime of the $J^{\pi }=4^{+}$ second excited state of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ at 3.555-MeV excitation, which was formed via the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}\mathrm{}(t,p)\mathrm{}{}_{}{}^{18}{}_{}{}^{}\mathrm{O}_{}^{*}{}_{}{}^{}$ reaction at $E_t=3.1\mathrm{}$ MeV. The mean life is found to be $\tau =46.2\mathrm{}\pm 3$ psec, which corresponds to an $E2\mathrm{}$ strength for the 3.55 ($4^{+}$→ 1.98 ($2^{+}$) transition of ${\mathrm{}\left|M\right(E2\mathrm{}\left)\right|}^2=0.64\mathrm{}\pm 0.05$ Weisskopf units. This result is in good agreement with the predictions of recent theoretical calculations in the weak-coupling model, which identifies the lowest-lying $2^{+}$ and $4^{+}$ states as predominantly 2p-0h states based on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. The predictions of this model for mirror states ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$-${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$ are discussed. Additional data were obtained coincidentally for states of ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ formed via the competing ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}\mathrm{}(t,n)\mathrm{}{}_{}{}^{18}{}_{}{}^{}\mathrm{F}_{}^{*}{}_{}{}^{}$ reaction. The results, which are in good agreement with previous values, are: 0.937-MeV state, $\tau =65.4\mathrm{}\pm 5.3$ psec and for the 1.081-MeV state, $\tau =26.1\mathrm{}\pm 2.3$ psec.

• Received 18 September 1973

DOI:https://doi.org/10.1103/PhysRevC.9.38