A Novel Method To Distinguish Fissile From Non-Fissile Materials Using Linearly Polarized Gamma-Ray Beams

We have developed a novel method to distinguish fissile materials, such as those which may be used as fuel in a nuclear reactor or in a nuclear weapon, from non-fissile materials. Our method relies upon using a linearly-polarized $\gamma$-ray beam to induce fission within a sample and then measuring the outgoing fission neutrons. The High Intensity $\gamma$-ray Source (HI$\gamma$S) generated the $\gamma$-ray beams used in our experiments designed to test this novel method. The HI$\gamma$S beam is quasi-monoenergetic and nearly 100\% linearly polarized. We performed photofission experiments using beams from 5.3 to 7.6 MeV on a variety of actinides: $^{232}$Th, $^{233,235,238}$U, $^{237}$Np, and $^{239,240}$Pu. In the fission process, on average 2-4 neutrons are emitted almost simultaneously with the fission event itself; these are known as prompt fission neutrons. An array of 12-18 liquid scintillator neutron detectors was used to measure the ratio of prompt fission neutron yields parallel to the plane of beam polarization to the yields perpendicular to this plane as a function of beam energy. A ratio near one was found for photofission of $^{233,235}$U, $^{237}$Np, and $^{239}$Pu while a significant ratio ($\sim$1.5-3) was found for $^{232}$Th, $^{238}$U, and $^{240}$Pu. This large difference could be used to distinguish fissile isotopes (such as $^{233,235}$U and $^{239}$Pu) from non-fissile isotopes (such as $^{232}$Th, $^{238}$U, and $^{240}$Pu). These ratios are in agreement with a model based on prompt neutron emission in fission and previously measured fission fragment angular distributions.