Elastic photon scattering between 9.5 and 12 MeV inPb208andPb206

The elastic photon scattering cross sections of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ and ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ were measured at 90° and 135° in the energy range from 9.5 to 12 MeV with a tagged photon beam whose energy spread was about 125 keV. The ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ cross section rises monotonically with energy, and is consistent with a total photon interaction cross section which has a Lorentzian energy dependence with a peak cross section of 650 mb at 13.6 MeV and a width $\Gamma \simeq 3.8$ MeV. The ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ scattering cross section is larger and has some rapid variations with energy; there is a narrow extra peak near 10.04 MeV and there are abrupt increases in the cross section just below 10.6 and 11.3 MeV. The relative scattering observed at the two angles indicates that all of the scattering, including the rapid variations with energy, is dominated by dipole interactions. This dipole assignment for the fine structure is important for the proper interpretation of inelastic electron scattering by ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. Some of the observed fine structure in inelastic electron scattering must be dipole; the fine structure previously reported as being due to electric quadrupole excitation should be considered as tentative until the correct dipole contributions are included.