LLNL is actively involved in the development of advanced technologies for use in detecting threats in sea-going cargo containers, particularly the presence of hidden special nuclear materials (SNM). The “Nuclear Car Wash” (NCW) project presented here uses a high-energy (En≈3.5–7.0MeV) neutron probe to scan a container and then takes high-energy (Eγ⩾2.5MeV), β-delayed γ-rays emitted during the subsequent decay of any short-lived, neutron-induced fission products as a signature of fissionable material. The components of the proposed system (e.g. neutron source, gamma detectors, etc.) will be discussed along with data processing schemes, possible threat detection metrics and potential interference signals. Results from recent laboratory experiments using a prototype system at LLNL will also be presented.

The screening of sea-going cargo containers for highly enriched uranium (HEU) and other fissile material is a challenging problem. This is due in part to the cargo itself, which acts as an attenuator to any radiation that might signal its presence. In the nuclear car wash, β-delayed high-energy γ-rays following neutron-induced fission are utilized as this signal. The delayed γ-rays above 3MeV are highly penetrating and have energies above natural background radiation. In addition, the half-lives of most fission products emitting γ-rays at these energies are less than 160s, making it feasible to construct decay curves on a time scale which preserves the flow of commerce through the port. A particular goal of the project is to understand the rate of false alarms. To this end, experiments are underway to investigate possible interferences, and to understand variations in the overall γ-ray background. The experiments and preliminary results are discussed. Work performed under the auspices of the DOE by the UC LLNL W7405Eng4,UCRL-PROC-224803.

The goal of any alarm algorithm should be that it provide the necessary tools to derive confidence limits on whether the existence of fissile materials is present in cargo containers. It should be able to extract these limits from (usually) noisy and/or weak data while maintaining a false alarm rate (FAR) that is economically suitable for port operations. It should also be able to perform its analysis within a reasonably short amount of time (i.e. {approx} seconds). To achieve this, it is essential that the algorithm be able to identify and subtract any interference signature that might otherwise be confused with a fissile signature. Lastly, the algorithm itself should be user-intuitive and user-friendly so that port operators with little or no experience with detection algorithms may use it with relative ease. In support of the Nuclear Car Wash project at Lawrence Livermore Laboratory, we have developed an alarm algorithm that satisfies the above requirements. The description of the this alarm algorithm, dubbed ALARMA, is the purpose of this technical report. The experimental setup of the nuclear car wash has been well documented [1, 2, 3]. The presence of fissile materials is inferred by examining the {beta}-delayed gamma spectrum induced after a brief neutron irradiation of cargo, particularly in the high-energy region above approximately 2.5 MeV. In this region naturally occurring gamma rays are virtually non-existent. Thermal-neutron induced fission of {sup 235}U and {sup 239}P, on the other hand, leaves a unique {beta}-delayed spectrum [4]. This spectrum comes from decays of fission products having half-lives as large as 30 seconds, many of which have high Q-values. Since high-energy photons penetrate matter more freely, it is natural to look for unique fissile signatures in this energy region after neutron irradiation. The goal of this interrogation procedure is a 95% success rate of detection of as little as 5 kilograms of fissile material while retaining at most .1% false alarm rate. Plywood is used to simulate hydrogenous cargo material and steel (pipes) is used to simulate metallic cargo. The wood consists of 120 x 240 cm sheets and has approximately .65 g/cm{sup 3}. The steel pipes have approximately 10 cm diameters x 6.4 mm wall thickness are .6 g/cm{sup 3}. Fissile sources consist of a ''large'' (380 g) and ''small'' (250 g) sample of HEU (U{sub 3}O{sub 8} 94% enriched). Note that the masses of the HEU sources used in our experimental runs are at least an order of magnitude smaller than 5 kilograms. Runs are done with either wood or steel cargoes stacked at various heights and the HEU sources placed at various depths within the 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.