Isovector-E2 Resonance In Pb-208

Polarization asymmetries observed in elastic tagged photon scattering at excitations between 16 and 30 MeV were used to identify the isovector giant electric quadrupole resonance in Pb-208 through its interference with underlying electric dipole strength. Resonance parameters reflecting the distribution of the E2 (T = 1) transition strength were extracted in a manner that has minimal sensitivity to uncertainties in the Delbruck amplitudes or in the q dependence of the modified Thomson amplitudes. The isovector E2 resonance was found at an excitation of 20.2 +/- 0.5 MeV, with a width of 5.5 +/- 0.5 MeV, and a strength corresponding to 1.4 +/- 0.3 times the isovector quadrupole sum rule.