Mean Life of the 2.98-MeV Level ofAl47

Nuclear-resonance—fluorescence techniques have been used to measure the mean life of the 2.98-MeV level of ${\mathrm{Al}}^{27}$. The exciting $\gamma$ radiation was obtained by bombarding a 99.999% pure Al target with 5.00-MeV protons. Assuming the spin of the level to be $\frac{3}{2}$ and the decay to be 99% directly to the ground state, self-absorption of the resonance radiation gives a width $\Gamma =\left(96_{-15\mathrm{}}^{}{}_{}{}^{+14\mathrm{}}\right)\times {10}^{-3\mathrm{}}$ eV. A scattering measurement gives $\Gamma =(105\mathrm{}\pm 15)\times {10}^{-3\mathrm{}}$ eV. The average is $\Gamma =(100\mathrm{}\pm 11)\times {10}^{-3\mathrm{}}$ eV, corresponding to a mean life $\tau =(6.6\mathrm{}\pm 0.7)\times {10}^{-15\mathrm{}}$ sec. The angular distribution for the resonance scattering was found to be $W\left(\theta \right)=1+(0.06\mathrm{}\pm 0.04)P_2-\left(cos\theta \right)$, which allows values of the $\frac{E2\mathrm{}}{M1\mathrm{}}$ amplitude ratio $-0.36<\delta <+0.20\mathrm{}$ and $-2.2<\delta <-1.1\mathrm{}$. The latter values are considered unlikely because of the large $E2\mathrm{}$ widths they imply. It was also ascertained that the 4.12-MeV level of ${\mathrm{Mg}}^{24}$ probably decays in less that ${10}^{-12\mathrm{}}$ sec.