The mean lifetime of the first excited state of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ at 0.32 MeV has been measured by the Doppler-shift-attenuation method. The result 0.18±0.06 psec corresponds to a very strong $E1$ transition, 0.33 W.u. This value is in very good agreement with a simple harmonic-oscillator picture of the $2s_{\frac{1}{2}}\to 1p_{\frac{1}{2}}$ transition which gives 0.32 W.u. However, the agreement is probably fortuitous, since the same model fails for the analogous transition in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, which is an order of magnitude too slow. No simple model relieves this discrepancy. The energy of the state in ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ is found to be 319.8±0.2 keV.

• Received 5 March 1971

DOI:https://doi.org/10.1103/PhysRevC.3.2198