Neutron emission asymmetries from linearly polarized $\gamma$ rays on $^{nat}$Cd, $^{nat}$Sn, and $^{181}$Ta

Azimuthal asymmetries in neutron yields produced by bombarding targets with linearly polarized photons via ($\gamma$,$n$), ($\gamma$,2$n$), and ($\gamma$,$f$) reactions are being investigated as a possible means of identifying various nuclear isotopes. The High Intensity $\gamma$-ray Source (HI$\gamma$S) at Duke University provides nearly monochromatic, circularly or linearly polarized $\gamma$ rays with high intensity by Compton backscattering free-electron-laser photons from stored electrons. Linearly polarized $\gamma$ rays produced by HI$\gamma$S were incident on $^{nat}$Cd, $^{nat}$Sn, and $^{181}$Ta targets at six energies $E_{\gamma}$ between 11.0 and 15.5 MeV and emitted neutrons were detected both parallel and perpendicular to the plane of polarization by an array of 18 liquid-scintillator detectors at angles in the range $\theta=55^{\circ}$--$142^{\circ}$. Detected neutrons were distinguished from Compton scattered photons by pulse-shape-discrimination and timing cuts, and their energies ($E_n$) were determined using time-of-flight information over a 0.5 m flight path. The characteristic plots of $R_n$, the ratio of neutron counts parallel to neutron counts perpendicular to the plane of the incident $\gamma$-ray polarization, against $E_n$ were constructed for each value of $E_{\gamma}$ and $\theta$ and then compared to those for other targets studied at HI$\gamma$S, including fissile nuclei $^{235}$U and $^{238}$U.