Conventional and Non-Conventional Nuclear Material Signatures

摘要

The detection and interdiction of concealed special nuclear material (SNM) in all modes of transport is one ofthe most critical security issues facing the United States and the rest of the world. In principle, detection of nuclearmaterials is relatively easy because of their unique properties: all of them are radioactive and all emit some characteristicgamma rays. A few emit neutrons as well. These signatures are the basis for passive non-intrusive detection of nuclearmaterials. The low energy of the radiations necessitates additional means of detection and validation. These are providedby high-energy x-ray radiography and by active inspection based on inducing nuclear reactions in the nuclear materials.Positive confirmation that a nuclear material is present or absent can be provided by interrogation of the inspected objectwith penetrating probing radiation, such as neutrons and photons. The radiation induces specific reactions in the nuclearmaterial yielding, in turn, penetrating signatures which can be detected outside the inspected object. The "conventional"signatures are first and foremost fission signatures: prompt and delayed neutrons and gamma rays. Their intensity(number per fission) and the fact that they have broad energy (non-discrete, though unique) distributions and certaintemporal behaviors are key to their use. The "non- conventional" signatures are not related to the fission process but tothe unique nuclear structure of each element or isotope in nature. This can be accessed through the excitation of isotopicnuclear levels (discrete and continuum) by neutron inelastic scattering or gamma resonance fluorescence. Finally there isan atomic signature, namely the high atomic number (Z > 74), which obviously includes all the nuclear materials andtheir possible shielding. The presence of such high-Z elements can be inferred by techniques using high-energy x rays.The conventional signatures have been addressed in another article. Non-conventional signatures and some of theircurrent or potential uses will be discussed here.