Polarized ($\gamma $,n) reaction studies of $^{nat}$Cd, $^{nat}$Sn, and $^{181}$Ta

Asymmetrical ($\gamma $,n) and ($\gamma $,f) neutron yields from polarized photons incident on fissile and non-fissile nuclei make it possible to construct isotopic signature curves useful for cargo interrogation. The High Intensity Gamma-Ray Source (HIGS) provides tunable, nearly mono-energetic $\gamma $ rays with high intensities and circular or linear polarization by colliding electrons with laser photons at the Duke Free Electron Laser Laboratory. HIGS was used to measure asymmetries in neutron emission from linearly polarized $\gamma $ rays incident on $^{nat}$Cd, $^{nat}$Sn, and $^{181}$Ta targets. An array of 18 liquid-scintillator detectors at six angles in the range $\theta $ = 55$^{\circ}$-142$^{\circ}$ was used to detect neutrons both parallel and perpendicular to the plane of polarization at six photon energies between 11.0 and 15.5 MeV. Since $^{nat}$Cd, $^{nat}$Sn, and $^{181}$Ta are non-fissile, any measured asymmetries resulted unambiguously from the ($\gamma $,n) reaction because the ($\gamma $,2n) threshold was above the incident photon energies. Neutrons were distinguished from Compton-scattered photons by pulse-shape discrimination and timing information, and their energies (E$_{n})$ were determined via time-of-flight techniques using a 0.5 meter flight path. The characteristic plots of the parallel/perpendicular ratio of neutron counts as a function of neutron energy E$_{n}$ were constructed and compared to those of previously studied targets at HIGS, including fissile nuclei such as $^{235}$U, $^{239}$Pu, and $^{232}$Th.