Volume 7, Issue 2
Container Strategy
by Paul Spinrad
Containerized shipping moves far more material across our borders than any other mode of transport. Each year, major ports in the United States handle thousands of cargo ships and millions of containers, and each container can carry up to 20 thousand kilograms of cargo. If just one or two kilos of highly enriched uranium were to make its way through this system, someone could use it to build a suitcase nuclear weapon. How do you detect the attempted smuggling of small amounts of fissionable material buried deep inside containers carrying lead bricks or any other legitimate cargo?
"We're looking for fissionable material rather than other radioactive material, which makes detection harder," explains nuclear engineering professor Eric B. Norman. "Highly radioactive Cobalt-60 or Cesium-137 can make a 'dirty bomb' that will scare people, but fissile materials like Uranium-235 and Plutonium-239 are both tougher to spot and far more potentially dangerous."
Norman's research group, a collaboration between UC Berkeley and Lawrence Livermore National Laboratory, uses a process called "active interrogation" to sniff out hidden fissile materials. Radioactive materials have traditionally been detected passively, as with Geiger counters, but active interrogation bombards the target with neutrons and then analyzes what comes out as a result. Previous methods measured the number of neutrons produced, but analyzing the resulting gamma rays provides a more detailed picture: A burst of gamma rays with very high energy levels (2.6 MeV and higher) indicates that the material is either fissile or heavy in sulfur or calcium. If this gamma radiation falls off with time, following a characteristic profile, that confirms that the material is fissile.
In practical terms, each cargo container must be tested in one minute to prevent backing up port operations. So Norman's group has prototyped a "nuclear car wash" at Livermore Lab that scans containers as they roll through on tracks. At the entrance, an underground particle accelerator shoots an upward stream of deuterium ions through a deuterium gas target. This creates helium and generates a beam of neutrons that shoots into the container. The level of neutron irradiation produced is comfortably below anything that could trigger a dangerous chain reaction in a fissionable target and, while it can briefly turn some cargo slightly radioactive, the effect soon dissipates.
"We haven't seen any showstoppers yet, and we aren't giving anyone a harmful dose of radiation," says Norman.
Flanking the container's path in the nuclear car wash are detector arrays with plastic scintillators that glow in response to gamma radiation. Photomultiplier tubes amplify this glow, and light detectors capture it for measurement.
Norman's group is testing the system on samples of highly enriched uranium (HEU) buried in big stacks of plywood, whose high hydrogen content characterizes many types of cargo and also absorbs neutrons, making detection more difficult. Conducting such experiments requires enormous administrative expertise in addition to scientific knowledge, since HEU samples are controlled by a complex and highly redundant system of security procedures and safeguards. Only one person on the team is authorized to physically handle the material, and all such handling must be logged. When the researchers want to move the sample into a different stack of plywood, they need to call the handler. All testing takes place at Livermore Lab, which is one of very few sites authorized to possess kilogram amounts of HEU.
Within the next year, Norman expects the Department of Homeland Security, which currently funds the research, to hold a Grand Challenge–style competition to decide which cargo scanning techniques should be fielded in U.S. ports. In the long run, multiple techniques will be used together to make up for each one’s weaknesses. If there's any doubt, a container will be shunted aside for detailed, open-lid inspection.
"We have to assume that the bad guys are as smart as we are," Norman explains, "and maybe they can fool the different techniques individually. But we can make them slip up trying to evade all of them at once. Only one of them has to work."
Find this article at:
http://www.coe.berkeley.edu/labnotes/3.07/container.html