Potential of x-ray diffraction for detecting Special Nuclear Materials (SNMs)

A novel detection technique employing x-ray diffraction (XRD) to screen for Special Nuclear Materials (SNMs), in particular for uranium, is presented. 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. Following a brief review of the cubic crystal structures exhibited by high atomic number species such as lead, the departure from cubicity exhibited by room-temperature uranium (orthorhombic) is discussed and its effect on the uranium XRD pattern is examined. The XRD lines of uranium are compared with those of lead, a common high-Z material found in container traffic. Significant differences are evident arising from their different crystal structures. In order to achieve adequate penetration, both of suspicious high-Z materials and their containers, high photon energies must be used. Physical and technological considerations relevant to performing XRD at 1 MeV are discussed and 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.