Compound and precompound effects in photon decay of the giant dipole resonance

We have investigated the excitation and ground-state photon decay of the giant resonance region in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ due to inelastic scattering of 84 MeV/nucleon ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$. The inelastic spectra are dominated by the isovector giant dipole resonance. The spectra of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ are almost identical in the giant resonance region. We conclude that the resonance strength functions are very nearly the same for these nuclei. Significant differences are, however, found in the distribution of ground-state decay photons from the two nuclei. These differences appear to be due to a large contribution from decay of fully damped compound states in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$.