This article details what Lawrence Livermore National Laboratory is doing in order to develop imporved technologies for detecting hidden radioactive materials in cargo containers that arrive at U.S. ports. "The Livermore system would bathe suspicious containers in neutrons to actively search for nuclear materials. A truck carrying a container laden with suspicious cargo would be towed over a generator that would bombard the container with neutrons. It would then be towed through an array of detectors, much like driving through a car wash. Neutrons encountered any fissile material shielded and hidden among the container's contents-whether produce, clothing, electronics, lumber, automotive parts, or other consumer goods-the interaction would induce tiny fission reactions. These reactions would produce the telltale delayed gamma rays of nuclear materials, which would be picked up by the detectors."
Lawrence Livermore National Laboratory: http://www.llnl.gov/