Giant resonances observed in the scattering of 96- and 115-MeV alpha particles

Beams of 96- and 115-MeV $\alpha$ particles were used to study the giant resonance region in 27 nuclei ranging from ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ to ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}.\mathrm{}$ A prominent broad ($\Gamma =3\mathrm{}-7$ MeV) peak is present in the spectra at an excitation energy of $\sim \frac{63}{A^{\frac{1}{3}}}$ MeV for all nuclei studied with $A\ge 36$. No such prominent peak is apparent for $A<\mathrm{}32\mathrm{}$. The angular distributions of the peak are characterized by an oscillatory behavior which is quite different from the monotonic behavior of the background. Distorted wave calculations with $L=2\mathrm{}$ reproduce the experimental angular distributions quite well for most of the nuclei. Energy weighted-sum rule fractions range from 30-50% for the lighter targets to near 100% for the heavier targets. Distorted wave calculations with a breathing mode ($L=0\mathrm{}$) form factor fail to reproduce the observed resonance strength by about a factor of 3 for all targets. The width of the peak is smallest for the closed shell nuclei ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. A small but significant broadening is observed in the rare-earth region of deformed nuclei. The giant dipole resonance is not appreciably excited for any of the targets.