Partial Photoneutron Cross Sections for 207,208Pb

Using linearly-polarized laser-Compton scattering γ-rays, partial E1 and M1 photoneutron cross sections along with total cross sections were determined for 207,208Pb at four energies near neutron threshold by measuring anisotropies in photoneutron emission. Separately, total photoneutron cross sections were measured for 207,208Pb with a high-efficiency 4π neutron detector. The partial cross section measurement provides direct evidence for the presence of pygmy dipole resonance (PDR) in 207,208Pb in the vicinity of neutron threshold. The strength of PDR amounts to 0.32%−0.42% of the Thomas-Reiche-Kuhn sum rule. Several units of B(M1)↑ strength were observed in 207,208Pb just above neutron threshold, which correspond to M1 cross sections less than 10% of the total photoneutron cross sections.