Prompt Neutron Polarization Asymmetries in Photossion of Isotopes of Thorium, Uranium, Neptunium, and Plutonium
Nearly mono-energetic, high intensity (~10<super>7</super> 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 <super>232</super>Th, <super>233,235,238</super>U, <super>237</super>Np, and <super>239,240</super>Pu. 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 (<super>233,235</super>U, <super>237</super>Np, and <super>239</super>Pu), while significant asymmetries were found for the even-even actinides (<super>232</super>Th, <super>238</super>U, and <super>240</super>Pu). 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.
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