Heavy-ion Coulomb excitation and photon decay of the giant dipole resonance inPb208

The excitation and electromagnetic decay of the giant resonance region (8–15 MeV) in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ have been measured with the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ${(}^{17}$O${,}^{17}$O’) reaction at 84 MeV/nucleon. The study of coincidences between scattered ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ ions and photon decays to the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ground state, including angular correlations, serves to isolate the isovector giant dipole resonance and enable us to investigate the mechanisms of its excitation and decay in detail. The angular correlations and yields are accounted for quantitatively by a pure Coulomb excitation model of the reaction process. The distribution in energy of the ground-state photon decay cross section is well described by an approximate application of the multistep theory of nuclear reactions. The photon coincidence measurements enable us to extract the differential cross sections for the 10.6-MeV giant quadrupole and 13.9-MeV monopole resonances with low uncertainty. An analysis of the Coulomb-nuclear interference for the 10.6-MeV excitation indicates that the ratio of neutron to proton matrix elements, ${\mathit{M}}_{\mathit{n}}$/${\mathit{M}}_{\mathit{p}}$, is 1.7±0.4, consistent with a predominantly isoscalar character for the giant quadrupole resonance.