Electroexcitation of odd-parity states inAl27

Inelastic electron scattering form factors were measured for the low-lying odd-parity states of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ over a momentum transfer range of 0.57 to 2.41 ${\mathrm{fm}}^{-1\mathrm{}}$. Reduced ground-state transition probabilities were deduced for states at $4.055 \mathrm{MeV}\left(\frac{1}{2^{-}}\right)$, $5.156 \mathrm{MeV}\left(\frac{3}{2^{-}}\right)$, $6.159 \mathrm{MeV}\left(\frac{3}{2^{-}}\right)$, $6.477 \mathrm{MeV}\left(\frac{7}{2^{-}}\right)$, $6.605 \mathrm{MeV}(\frac{1}{2^{-}} or \frac{3}{2^{-}})$, $6.651 \mathrm{MeV}\left(\frac{5}{2^{-}}\right)$, $6.993 \mathrm{MeV}\left(\frac{3}{2^{-}}\right)$, and $7.228 \mathrm{MeV}\left(\frac{9}{2^{-}}\right)$. In addition, the results support odd-parity assignments for the $5.827 \mathrm{MeV}(\frac{3}{2^{-}} or \frac{5}{2^{+}})$ and $7.477 \mathrm{MeV}\left(\frac{7}{2}\right)$ levels. Attempts have been made to interpret the results in terms of the weak-coupling and strong-coupling models. The measurements confirm the apparent high concentration of $1p$-shell proton hole strength in the 4.055 MeV,$\frac{1}{2^{-}}$ and 5.156 MeV,$\frac{3}{2^{-}}$ states. To satisfactorily account for the observed properties of these two levels using the weak-coupling model requires an anomalously small value of 1.35 MeV for the spin-orbit splitting of the $1p$ shell. The structure of levels in the 6 to 7.5 MeV region is shown to be based on the excitation of $1d$-shell nucleons into the $1f$ shell.