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November 26, 2003
A Better Way To Detect Nuclear Contraband

A new technique that might lead to the quick, accurate and safe detection of nuclear materials smuggled inside sea-going cargo containers has been demonstrated by a team of researchers with Lawrence Berkeley National Laboratory, the University of California at Berkeley, and the Lawrence Livermore National Laboratory.

At U.S. seaports such as this one in Miami, nearly 7,000,000 cargo containers are offloaded each year. A container may hold about 20 metric tons of goods and represents a prime target of opportunity for smugglers.

Using Berkeley Lab's 88-Inch Cyclotron, the researchers showed that irradiating a cargo container's contents with a beam of neutrons, then measuring the emission of high-energy gamma rays, provides a unique identifying signature of plutonium or highly enriched uranium that can be used for weapons of mass destruction. A detection system based on this technology has the potential to screen every cargo container entering a United States seaport.

"Our method is based on the fact that neutron-induced fission of special nuclear materials is followed by beta decays of short-lived fission fragments during which large numbers of high-energy gamma rays [above 3 million electron volts] are emitted," the authors write in a paper that will appear in the journal Nuclear Instruments and Methods in Physics Research.

"These gamma rays have energies above those of natural background, are emitted with significantly greater intensity than beta-delayed neutrons, have much higher probabilities of escaping hydrogenous cargo loadings than neutrons, and their energy spectra and time dependencies provide a unique signature of special nuclear materials," the authors write.

Stan Prussin (left) and Rick Norman have demonstrated that high-energy gamma-ray emissions can reveal the presence of weapons-grade nuclear materials hidden in cargo containers.

Coauthoring the scientific paper were Eric Norman, of Berkeley Lab's Nuclear Science Division, and Stanley Prussin, with UC Berkeley's Nuclear Engineering Department, plus Ruth-Mary Larimer, Edgardo Brown, Alan Smith, Richard McDonald and Heino Nitsche, of Berkeley Lab, Howard Shugart and Puja Gupta of UC Berkeley, and Michael Frank and Thomas Gosnell of Livermore.

Homeland security experts say that terrorist threats are most likely to enter our nation by way of the sea, in one of the nearly seven million cargo containers offloaded at U.S. ports every year. These tractor-trailer-sized, steel-walled boxes are typically sealed shut in foreign ports and not opened until delivered by trucks to points all across the country.

Despite heightened security concerns following the September 11 terrorist attacks, less than two percent of these containers are ever opened and inspected at U.S. seaports, according to the U.S. Customs and Border Protection agency. It is widely agreed that improved technologies for the nonintrusive inspection of cargo containers are sorely needed. Of greatest concern is the search for the so-called special nuclear materials, such as plutonium or uranium 235. Although only mildly radioactive, special nuclear materials in concentrated form can serve as the primary ingredients of nuclear explosives.

"It only takes about 10 kilograms of special nuclear materials to make an atomic bomb, which is about enough material to stuff inside a baseball," says Norman. "You have to be able to find this when it's hidden in a container that might be holding nearly 20 metric tons of cargo."

Whereas x-ray imaging can reveal the presence of an object inside a container that does not conform to a declared manifest, Norman and Prussin say that detecting the emission of high-energy gamma rays gives "a definitive yes or no" as to whether the smuggled object contains special nuclear materials.

Norman and Prussin got their idea for using gamma-ray emissions to detect the presence of special nuclear materials while serving as consultants on a project at Livermore led by Dennis Slaughter, technical director of the 100 MeV Electron-Positron Linac Facility. Livermore researchers were exploring the possibility of bombarding a cargo container with energized neutrons that would trigger a brief fission reaction in plutonium and enriched uranium, while not significantly affecting elements such as hydrogen, carbon, oxygen, or calcium, which make up food products and other materials commonly shipped by sea.

Among the emission products from this fission reaction are "delayed neutrons" — those slowly emitted following an initial burst of released neutrons — which the Livermore researchers were looking to measure. Norman and Prussin were concerned that a heavy presence of hydrogen in a cargo container would present a severe roadblock to measuring neutron emissions, as hydrogen readily absorbs delayed neutrons.

"Hydrogen is all over the place in containers," says Prussin. "We import fruits and vegetables, filled with water. Computers? They're made of plastic, predominantly composed of carbon and hydrogen. However, there are other emission products less prone to absorption in hydrogenous materials. Our thoughts turned to high-energy gamma rays, which can penetrate through hydrogenous materials a heck of a lot better than delayed neutrons can."

Calculations showed that the relative intensity of delayed high-energy gamma rays is approximately 10 times greater than delayed neutrons and that gamma rays would get through hydrogenous materials anywhere from 100 to 1,000 times more easily. This means that under some circumstances, gamma rays would be as much as 10,000 times more sensitive a means of detecting special nuclear materials than delayed neutrons.

As a proof-of-principle experiment, Norman and Prussin and their Berkeley and Livermore collaborators generated 8-million-electron-volt beams of neutrons at the 88-Inch Cyclotron, which they moderated and then used to irradiate sample targets of plutonium 239 and uranium 235, plus a variety of materials that might be found in a typical cargo container including wood, polyethylene, aluminum, and steel. They then measured gamma-ray emissions, using both a high-resolution germanium detector and a low-resolution plastic scintillator.

At Berkeley Lab's 88-Inch Cyclotron the researchers irradiated materials found in typical cargo containers, plus sample targets of plutonium 239 and uranium 235, with beams of 8-MeV neutrons.

"We found that gamma rays were emitted in a wedge-shaped energy spectrum that is well above any normal background that may be present during the screening process," Norman says. "We also saw a characteristic half-life decay of approximately 25 seconds. All other materials showed much longer decay times."

Based on their tests at the 88-Inch Cyclotron, the collaboration concluded that with this technology an entire cargo container might be screened for the presence of special nuclear materials in about one minute. Using relatively inexpensive plastic scintillators to detect the gamma rays is every bit as effective as the more costly germanium detectors, which helps hold down the cost of a detection system and also speeds up the process. The next step calls for experiments using a full-size container that's packed with mock cargo and a hidden sample of special nuclear material. These experiments are already being planned to take place at Livermore Lab's test facilities.

Norman says a full-scale system for use at a major seaport could be built at an economically practical cost. While tests at Livermore are needed to determine whether the technology will be effective for use in seaports, Prussin is optimistic. He says, "I think this method has a much higher probability of success than anything that's been suggested so far."

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