Primary Screening of Cargo Containers for high-Z Materials
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: Abstract - This proposal addresses the pressing problem of providing a general screening methodology that can detect high atomic number material (Z ≥ 72) in elemental form that might be embedded in the massive cargos of intermodal containers. Such an approach would greatly minimize the number of containers suspected of housing significant quantities of clandestine fissionable materials, localize possible targets of interest and then permit definitive searches when warranted by irradiation with very high-energy γ rays or high-energy neutrons to produce fission. It has been shown that high-energy β-delayed γ rays are a unique signature of fission that can easily be detected even in the presence of massive shielding. We recently reported results from Monte Carlo simulations that demonstrate a radiographic methodology has the potential to identify quantities as small as 0.1 kg of uranium (U) or plutonium (Pu) metal as having Z ≥ 72 in intermodal containers filled homogeneously with normal cargo at the maximum allowable density. This would exceed the sensitivity requirements set by the Department of Homeland Security (DHS) for the Cargo Automated Advanced Radiography System (CAARS) for which recent commercial concepts have failed. The proposed methodology is centered on the measurement of linear attenuation coefficients with sufficient accuracy to categorize the effective atomic number of an object as Z ≤ 72 or Z ≥ 72 within the false-positive and false-negative limits set by the DHS. It is grounded in the fundamentals of incoherent scattering, the rather high spatial resolution afforded by the interaction of high-energy electrons in a practical detector and the near constancy of mass attenuation coefficients of 3-9 MeV photons for the elements that control the contents of most cargos. It has the potential to provide for a simple, rapid procedure that produces no activation of the cargo itself. The detector system would be sensitive to the β-delayed γ rays that are a robust signature of fission. Thus, a dual-purpose screening/definitive interrogation system would be possible that would provide a means for limiting activation of cargos by such "active" interrogation approaches to localized volumes. Target localization would also simplify and facilitate the use of nuclear resonance fluorescence technologies now being investigated scientifically. The methodology would make use of commercial bremsstrahlung sources well within their reported performance characteristics and utilize commercially-available detector materials. The proposal seeks to extend our preliminary work with Monte Carlo simulations to define the limits of the fundamental physics assumptions on which it is based for realistic cargo configurations, including inhomogeneous media with regular and randomly-spaced voids. It also will examine whether the methodology can be extended to non-metallic sources with any reliability. It seeks to design and construct two detector arrays composed of materials chosen to span the range of scintillators likely to be used in practice and use the arrays in experiments with various targets of fissionable materials and surrogates. We envision these tests to be accomplished blind to project personnel, followed by data analysis and evaluation to test the practicalities of the methodology and systems as realistically as possible. Close collaboration between experts at LLNL and Berkeley over the period FY 2004-2007 contributed to the successful demonstration of the use of neutron interrogation and β-delayed γ rays following fission for detecting relatively small quantities of fissionable materials embedded in massive shielding. If funded, this proposal would allow for a continued collaboration of LLNL and Berkeley experts in a similar manner. It leverages funding in place from a DHS ARI grant to the University of California, Berkeley that explicitly prohibits any funding of Laboratory personnel or use of Laboratory facilities on a cost-for-service basis. The proposal would leverage the world-class computing facilities at LLNL and the radiography facility that can handle both classified and surrogate targets. It would support an additional graduate student.