Prompt neutron polarization asymmetries in photofission of 232Th, 233,235,238U, 237Np and 239,240Pu.

Photofission experiments were performed on targets of 232Th, 233,235,238U, 237Np, and 239,240pu using nearly 100% linearly polarized, high intensity (~5 x 106γ/s), and nearly-monoenergetic γ-ray beams having energies between 5.3 and 7.6 MeV at the High Intensity γ-ray Source (HlγS) located at Duke University and Triangle Universities Nuclear Laboratory. An array of 12-18 liquid scintillator detectors was used to measure prompt fission neutron yields parallel and perpendicular to the plane of beam polarization. Polarization asymmetries, the differences between the in-plane and out-of-plane yields divided by their sums, were measured. Asymmetries close to zero were found for 233,235U, 237Np, and 239Pu while significant asymmetries (~0.2-0.5) were found for 232Th, 238U, and 240Pu. Predictions based on previously measured photofission fragment angular distributions combined with a model of prompt neutron emission agree well with the experimental results. [ABSTRACT FROM AUTHOR]