Screening of Maritime Containers to Intercept Weapons of Mass Destruction

Abstract

The goal of our research was to address the problem of detection of weapons of mass destruction (WMD) materials within containers in common use on commercial cargo trafficking. LLNL has created an experimental test bed for researching potential solutions using (among other techniques) active interrogation with neutrons. Experiments and computational modeling were used to determine the effectiveness of the technique.Chemical weapons materials and high explosives can be detected using neutron activation and simple geometries with little or no intervening material. However in a loaded container there will be nuisance alarms from conflicting signatures resulting from the presence of material between the target and the detector (and the interrogation source). Identifying some elements may require long counting times because of the increased background. We performed some simple signature measurements and simulations of gamma-ray spectra from several chemical simulants. We identified areas where the nuclear data was inadequate to perform detailed computations.We concentrated on the detection of SNM in cargo containers, which will be emphasized here. The goal of the work reported here is to develop a concept for an active neutron interrogation system that can detect small targets of SNM contraband in cargo containers, roughly 5 kg HEU or 1 kg Pu, even when well shielded by a thick cargo. It is essential that the concept be reliable and have low false-positive and false-negative error rates. It also must be rapid to avoid interruption of commerce, completing the analysis in minutes.A potentially viable concept for cargo interrogation has been developed and its components have been evaluated experimentally. A new radiation signature unique to SNM has been identified that utilizes high-energy, fission-product gamma rays. That signature due to g-radiation in the range 3-6 MeV is distinct from normal background radioactivity that does not extend above 2.6 MeV. It's short half-life of 20-55 sec makes it distinct from neutron activation due to the interrogation that is typically much longer lived. This work spawned a collaboration with LBNL where experiments verified the abundance and other characteristics of this new signature [24]. Follow-on work funded by DoE/NA22 led to the development of a detailed system concept and evaluation of its impact on operating personnel and cargos [60] and characterization of one important interference that was identified [61]. The follow-on work led to two patent applications at LBNL and LLNL. The signature flux, while small, is 2-5 decades more intense than delayed neutron signals used and facilitates the detection of SNM even when shielded by thick cargo. The actual benefit is highly dependent on the type and thickness of cargo, with modest benefit in the case of metallic cargos of iron, lead, or aluminum, but maximum benefit in the case of hydrogenous cargo. In addition, unwanted collateral effects of the interrogation, such as neutron activation of the cargo, were analyzed [60] and one sign...