Total and partial photoneutron cross sections for Pb isotopes

Abstract

Using quasimonochromatic laser-Compton scattering $\ensuremath{\gamma}$ rays, total photoneutron cross sections were measured for ${}^{206,\phantom{\rule{0.16em}{0ex}}207,\phantom{\rule{0.16em}{0ex}}208}$Pb near neutron threshold with a high-efficiency 4$\ensuremath{\pi}$ neutron detector. Partial $E1$ and $M1$ photoneutron cross sections along with total cross sections were determined for ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb at four energies near threshold by measuring anisotropies in photoneutron emission with linearly polarized $\ensuremath{\gamma}$ 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 ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb near the neutron threshold corresponding to 0.32$%$--0.42$%$ of the Thomas-Reiche-Kuhn sum rule. Several ${\ensuremath{\mu}}_{N}^{2}$ units of $B(M1)\ensuremath{\uparrow}$ strength were observed in ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb just above neutron threshold, which correspond to an $M1$ cross section less than 10$%$ of the total photoneutron cross section.