Giant quadrupole resonance inMg24,Al27, andSi28

The giant-resonance region of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ was studied by inelastic scattering of 126-MeV α particles. In contrast to results at 96 MeV, considerable clustering of $E2\mathrm{}$ strength was observed for ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ at $E_x\sim 20.1$ MeV with $\Gamma \sim 7.6$ MeV exhausting about 35% of the $E2\mathrm{}$ energy weighted sum rule. $E2\mathrm{}$ strength was also located in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ in two clusters of states at ${\overline{E}}_x\sim 18.2, 24.4$ MeV; however, contributions from other multipoles cannot be neglected. In ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ a multipeaked group was observed at $E_x\sim 19.4$ MeV with $\Gamma \sim 4$ MeV but no $L$ assignment was made. The energy dependence of the cross section for the giant quadrupole resonance was found to be consistent with distorted-wave Born approximation predictions.