Photofission Neutron Yield Ratios on $^{238}{\rm U}$ near $E_{\gamma}=6.2$ MeV using Linearly Polarized $\gamma$ rays

Neutron yields and the ratios of the yields measured parallel to the plane of $\gamma$-ray polarization over the yields perpendicular to the plane of polarization ($I_{par}/I_{perp}$) have been measured using a U-238 target for the first time near the ($\gamma,n$) threshold of $E_\gamma \simeq 6.2$ MeV. Measurements were performed at $\gamma$-ray energies of 5.7 MeV (near the photofission threshold) through the ($\gamma,n$) threshold up to 6.5 MeV. The $I_{par}/I_{perp}$ data taken with the nearly 100\% linearly polarized beams at HI$\gamma$S have values ranging from 3 to 4 in the pure fission region below the ($\gamma,n$) threshold to about 2 at energies just above the ($\gamma,n$) threshold. In an effort to understand these new data, a model has been developed where the neutrons are emitted isotropically in the center-of-mass frame of the fission fragments. The fission fragment angular distributions are taken from previous $\gamma$-ray and neutron induced fission data and are used to predict the values of $I_{par}/I_{perp}$ for both the fission fragments and the neutrons. Experimental results will be shown and compared with the results of these calculations.