Using quasimonochromatic laser-Compton scattering $\gamma$ rays, total photoneutron cross sections were measured for ${}^{206,207,208}$Pb near neutron threshold with a high-efficiency 4$\pi$ neutron detector. Partial $E1$ and $M1$ photoneutron cross sections along with total cross sections were determined for ${}^{207,208}$Pb at four energies near threshold by measuring anisotropies in photoneutron emission with linearly polarized $\gamma$ rays. The $E1$ strength dominates over the $M1$ strength in the neutron channel where $E1$ photoneutron cross sections show extra strength of the pygmy dipole resonance in ${}^{207,208}$Pb near the neutron threshold corresponding to 0.32$\%$–0.42$\%$ of the Thomas-Reiche-Kuhn sum rule. Several ${\mu }_N^2$ units of $B\left(M1\right)\uparrow$ strength were observed in ${}^{207,208}$Pb just above neutron threshold, which correspond to an $M1$ cross section less than 10$\%$ of the total photoneutron cross section.

• Received 12 June 2012

DOI:https://doi.org/10.1103/PhysRevC.86.014316