Speed of the $\Delta J=1\mathrm{}$, $\Delta T=1\mathrm{}$ $M1\mathrm{}$ Transition in ${\mathrm{Al}}^{26}$

The attenuated-Doppler-shift technique has been used to determine the mean life of the 1.059-MeV third excited state in ${\mathrm{Al}}^{26}$ to be (${3.1}_{-0.8\mathrm{}}^{+1.1\mathrm{}}$)×${10}^{-14\mathrm{}}$ sec. The state decays primarily to the $J^{\pi }=0^{+}$, $T=1\mathrm{}$, 0.229-MeV first excited state. The spin component in this pure $M1\mathrm{}$ transition can be deduced from the analogous Gamow-Teller $\beta$ decay from the ${\mathrm{Si}}^{26}$ ground state to the 1.059-MeV state in ${\mathrm{Al}}^{26}$. The observed lifetime is only about half what would be predicted from the spin component alone. This indicates a strong orbital contribution which, in turn, implies a large ${\left(d_{\frac{5}{2}}\right)}^{-2\mathrm{}}$ component in the wave function. A summary of $M1\mathrm{}$ transitions whose spin components are known from analogous $\beta$ decays is given. A mean life of (4.8±2.5)×${10}^{-14\mathrm{}}$ sec was found for the second excited state of ${\mathrm{Mg}}^{26}$.