Prompt Neutron Polarization Asymmetries in Photossion of Isotopes of Thorium, Uranium, Neptunium, and Plutonium

Nearly mono-energetic, high intensity (~107 gamma/s), and approximately 100% linearly polarized gamma-ray beams at energies between 5.15 and 7.60 MeV were used to induce photofission of 232Th, 233,235,238U, 237Np, and 239,240Pu. Prompt fission neutron yields parallel and perpendicular to the plane of beam polarization were measured using arrays of 12-18 liquid scintillator detectors. Prompt neutron polarization asymmetries close to zero were found for the even-odd actinides (233,235U, 237Np, and 239Pu), while significant asymmetries were found for the even-even actinides (232Th, 238U, and 240Pu). Predictions based on previously measured fission fragment angular distributions combined with a model of prompt neutron emission agree well with our experimental results. Finally, we describe a new method of measuring the enrichment of special nuclear material based on our results.