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

Abstract

Beams of 96- and 115-MeV $alpha$ particles were used to study the giant resonance region in 27 nuclei ranging from $sup 14$N to $sup 208$Pb. A prominent broad (GAMMA=3--7 MeV) peak is present in the spectra at an excitation energy of approx.63/A$sup 1$/$sup 3$ MeV for all nuclei studied with A> or =36. No such prominent peak is apparent for A<32. 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 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) 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 $sup 40$Ca, $sup 90$Zr, $sup 144$Sm, and $sup 208$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. (AIP)