Photon scattering in the giant resonance region of Mg24, Si28, and S32

Cross sections for the elastic and inelastic scattering of monochromatic photons from Mg24, Si28, and S32 have been measured for incident photon energies between 17 and 28 MeV, spanning the region of the giant dipole resonance. The cross sections are interpreted in terms of the coupling between the giant dipole resonances built on the ground and 21+ states. A good quantitative description of the energy-averaged cross sections is obtained in the context of the dynamic collective model, provided S32 is a vibrational nucleus and Mg24 and Si28 are prolate and oblate nuclei, respectively. The cross sections for Si28 show considerable structure that is correlated in the elastic and 21+ channels and that is not predicted by the dynamic collective model. The elastic cross section is used to infer the total photoabsorption cross section, which is shown to be somewhat more structured than implied by previous direct measurements.