Analysis of Recent Manifests for Goods Imported through US Ports

Several active interrogation techniques are being developed to detect shielded special nuclear materials (SNM) hidden in cargo containers loaded on container ships arriving at US ports. It raises the questions of the types of cargos in which SNM could be hidden, and their impact on detected signatures. Since the definition of a set of ''typical'' or standard cargos has proven to be difficult, we analyzed shipping manifests for US imports shipped through North American ports collected on 14 days distributed over 12 months. From these data, we generated several distribution functions such as commodity categories, average densities, and packaging types that could be of interest to the cargo scanning community. One of the cargo scanning techniques currently under development at LLNL is based on neutron active interrogation, and relies on the unique signature of beta-delayed gammas emitted by fission products in the 3 to 7 MeV energy range. {sup 19}F(n,a){sup 16}N, has been identified as the main potential interference for 7 and 9 MeV neutron beams. Estimates of cargo compositions based on manifests identified as containing fluorine are presented

Test Suite for Nuclear Data I: Deterministic Calculations for Critical Assemblies and Replacement Coefficients

The authors describe tools developed by the Computational Nuclear Physics group for testing the quality of internally developed nuclear data and the fidelity of translations from ENDF formatted data to ENDL formatted data used by Livermore. These tests include S{sub n} calculations for the effective k value characterizing critical assemblies and for replacement coefficients of different materials embedded in the Godiva and Jezebel critical assemblies. For those assemblies and replacement materials for which reliable experimental information is available, these calculations provide an integral check on the quality of data. Because members of the ENDF and reactor communities use calculations for these same assemblies in their validation process, a comparison between their results with ENDF formatted data and their results with data translated into the ENDL format provides a strong check on the accuracy of translations. As a first application of the test suite they present a study comparing ENDL 99 and ENDF/B-V. They also consider the quality of the ENDF/B-V translation previously done by the Computational Nuclear Physics group. No significant errors are found

Spectra Observed Following Cargo Interrogation

The authors present calculations of photon spectra observed following irradiation of bare HEU, HEU embedded in steel and wood cargos, and steel and wood alone. These spectra might be useful starting points for statistical detection efforts aimed at determining whether fissile material is present in a cargo. Detailed comparisons between calculations and experiments are presented and overall quite good (small {chi}{sup 2}) agreement is found. they do not present a complete solution to the problem of determining whether a given spectrum contains contributions from post-fission photons. However, it is shown that a brute-force fitting of observed spectra in terms of a few calculated ''basis'' spectra gives meaningful predictions about the presence of {sup 235}U in cargo. Though this may not be the most powerful method, it does give well defined confidence limits and seems to have strong predictive power

Detector design for high-resolution MeV photon imaging of cargo containers using spectral information

Monte Carlo simulations of a pixelated detector array of inorganic scintillators for high spatial resolution imaging of 1-9 MeV photons are presented. The results suggest that a detector array of 0.5 cm x 0.5 cm x 5 cm pixels of bismuth germanate may provide sufficient efficiency and spatial resolution to permit imaging of an object with uncertainties in dimension of several mm. The cross talk between pixels is found to be in the range of a few percent when pixels are shielded by {approx} 1mm of lead or tungsten. The contrast at the edge of an object is greatly improved by rejection of events depositing less than {approx} 1 MeV. Given the relatively short decay time of BGO, the simulations suggest that such a detector may prove adequate for the purpose of rapid scanning of highly-shielded cargos for possible presence of high atomic number (including clandestine fissionable) materials when used with low current high duty factor x-ray sources

A Proposal for High-resolution X-ray Imaging of Intermodal Cargo Containers for Fissionable Materials

The sensitivity for identification of high-Z objects in elemental form in the massive cargo of intermodal containers with continuous bremsstrahlung radiation depends critically on discriminating the weak signal from uncollided photons from the very intense flux of scattered radiations that penetrate the cargo. We propose that this might be accomplished by rejection of detected events with E {le} 2-3 MeV that contain the majority of multiply-scattered photons along with a correction for single-scattered photons at higher energies. Monte Carlo simulations of radiographs with a 9-MeV bremsstrahlung spectrum demonstrate that rejection of detected events with E {le} 3 MeV removes the majority of signals from scattered photons emerging through cargo with Z {le} 30 and areal densities of at least 145 g cm{sup -2}. With analytical estimates of the single-scattered intensity at higher energies, accurate estimates of linear attenuation coefficients for shielded and unshielded uranium spheres with masses as small as 0.08 kg are found. The estimated maximum dose is generally so low that reasonable order tomography of interesting portions of a container should be possible

Neutron and Photon Transport in Sea-Going Cargo Containers

Factors affecting sensing of small quantities of fissionable material in large sea-going cargo containers by neutron interrogation and detection of {beta}-delayed photons are explored. The propagation of variable-energy neutrons in cargos, subsequent fission of hidden nuclear material and production of the {beta}-delayed photons, and the propagation of these photons to an external detector are considered explicitly. Detailed results of Monte Carlo simulations of these stages in representative cargos are presented. Analytical models are developed both as a basis for a quantitative understanding of the interrogation process and as a tool to allow ready extrapolation of the results to cases not specifically considered here

Nuclear Car Wash sensitivity in varying thicknesses of wood and steel cargo

The influence of incident neutron attenuation on signal strengths in the Nuclear Car Wash has been observed experimentally for both wood and steel-pipe mock cargos. Measured decay curves are presented for {beta}-delayed high-energy {gamma}-rays and thermalized neutrons following neutron-induced fission of HEU through varying irradiation lengths. Error rates are extracted for delayed-{gamma} and delayed-n signals integrated to 30 seconds, assuming Gaussian distributions for the active background. The extrapolation to a field system of 1 mA deuterium current and to a 5 kg sample size is discussed