Lifetimes of several low-lying levels in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$, and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ have been measured using the Doppler-shift attenuation method. The levels were populated by the $X(\alpha ,{\alpha }^{\prime })X^{*}$ reaction using 22-MeV $\alpha$ particles from the Indiana University cyclotron. The backscattered $\alpha$ particles were detected in an annular surface-barrier particle detector, and coincident $\gamma$ rays resulting from the decay of the nuclear states were detected by a 16.7-${\mathrm{cm}}^3$ lithium-drifted germanium detector. A new form for the velocity of the recoiling ions as a function of time, which is suitable for heavy target backings and high initial recoil velocities ($\sim 0.03c$), was used. The results for the mean lifetimes, in units of ${10}^{-15\mathrm{}}$ sec, for the excited states in parentheses (MeV) are the following: ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(4.12) 169±34, (4.23) 185±33, (5.23) 173±46, (6.01) 124±20; ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$(2.94) ${60}_{-20\mathrm{}}^{+12\mathrm{}}$; ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(4.61) ${83}_{-12\mathrm{}}^{+15\mathrm{}}$. These results are compared with previously reported values, and best averages are suggested.

• Received 15 March 1971

DOI:https://doi.org/10.1103/PhysRevC.4.461