Active Interrogation Using Energetic Protons

Active interrogation with sources of gamma rays and neutrons is being explored as a possible method for the inspection of various cargos for nuclear materials or devices.1 Active interrogation has been shown to increase the radiation signature of unshielded or lightly shielded nuclear material enormously. The delayed neutron and gamma signatures can uniquely establish the presence of fissionable and fissile material. The difficulty is that even this increased signal can be significantly reduced by heavily shielding the target. We discuss an alternative—proton interrogation, which fundamental principles suggest would provide a much better source. As an example, the load limit of a standard shipping container of about 20 metric tons allows shielding configurations that are very difficult to penetrate with conventional nuclear probes. Consider a shielded volume of 50 cm in radius and 100 cm long in the center of a 2.5×2.5×10 m3 container. Such a volume could be shielded with 10 cm of lead and 90 cm of polyethylene on all sides. The lead would provide gamma ray shielding, and the polyethylene, neutron shielding. An inner layer of borated polyethylene would absorb thermal neutrons, and the outer polyethylene would absorb a large fraction of the 0.4 MeV gamma rays emitted in the neutron capture. A schematic view of what such a shield might look like is shown in Figure 1. The weight of such a shield configured as hemispherical end caps on a 50-cm-long cylinder is calculated to be about 10 tons. When one considers that even the polyethylene shield (180 g/cm2) cannot be penetrated without very large x-ray doses. Intermediate energy protons (~1 Gev) have much longer attenuation lengths than the more conventional probes, and can penetrate thick cargoes. In addition they have large cross sections2 , 1.0 barn, for producing fission in actinide materials. Because of the obvious benefit of the high penetration of protons and much lower doses needed to produinterrogation.