On screening for Special Nuclear Materials (SNMs) with X-ray diffraction

A novel detection technique employing X-ray diffraction (XRD) to screen for Special Nuclear Materials (SNMs), in particular for uranium, has been recently proposed. It is based on the interesting fact that uranium (and incidentally, plutonium) has a non-cubic lattice structure, in contrast to all other non-SNM, high-density elements of the Periodic Table. The principle of this screening technique is briefly elucidated by comparing the XRD lines of uranium with those of lead, a material of high atomic number (Z) commonly found in container traffic.Several physical conditions that must be satisfied to enable XRD for SNM screening are considered. To achieve adequate penetration, both of suspicious high-Z materials and their containers, photon energies of 1 MeV and above must be employed. Implications from partial coherence theory for the XRD measurement geometry at such photon energies are presented. The question of multiple scatter degradation of the coherent scatter signal is addressed.Technological considerations relevant to performing XRD at 1 MeV, particularly regarding the radiation source and detector, are discussed. A novel secondary aperture scheme permitting high energy XRD is presented. It is concluded that the importance of the application and the prospect of its feasibility are sufficient to warrant experimental verification.