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Professor Stanley Prussin has been on the UC Berkeley faculty since 1966.

Each year, nearly seven million shipping containers pass through US ports. With tight time constraints allowing just two percent of the containers to be inspected, there is a very real fear that one of these twenty to forty foot long containers could be a Trojan horse hiding the key ingredient in a nuclear weapon. The challenge is that detecting a baseball-sized bit of highly-enriched uranium or plutonium buried inside 27 metric tons of fruit, furniture, or computers is like looking for a needle in a haystack without knowing if the needle is even there. To detect the clandestine transport of nuclear weapons materials, UC Berkeley nuclear engineering professor Stanley G. Prussin and Eric B. Norman, a senior nuclear scientist at Lawrence Berkeley National Laboratory (LBNL), are working with scientists at the Lawrence Livermore National Laboratory (LLNL) to develop a nuclear detection method that may be 10,000 times more sensitive under some conditions than other approaches currently being tested.

A ship carrying some of the 7 million shipping containers that pass through US ports each year. (courtesy Dennis Slaughter/LLNL)

Prussin and Norman's method involves bombarding a shipping container with a beam of neutrons that will induce a safe fission reaction if uranium or plutonium is inside the box. The researchers can then detect gamma rays produced by the reaction. The idea came to Prussin and Norman while they observed the cargo container screening effort at LLNL. The LLNL scientists, Prussin explains, also employed a beam of neutrons to induce the fission reaction. If fission occurs, the LLNL researchers hoped they could count "delayed neutrons," the radiation slowly emitted by a fission reaction after the initial burst of neutrons is released.

The difficulty, Prussin says, is that neutrons are likely to get absorbed by materials containing large quantities of hydrogen before they even make it out of the shipping container.

"We import fruits and vegetables, filled with water," Prussin says. Computers? They're made of plastic, predominantly composed of carbon and hydrogen. Hydrogen is all over the place."

This testbed at LLNL enables researchers to experiment with nuclear detection methods on a real cargo container. (courtesy Dennis Slaughter/LLNL)

It occurred to Prussin and Norman that there are other delayed radiations that may be less prone to absorption in hydrogenous materials. High-energy gamma rays, he says, "can penetrate through such matter a heck of a lot better than neutrons can."

After a few simple calculations, Prussin determined that the relative intensity of delayed high-energy gamma rays is approximately 10 times larger than delayed neutrons. Gamma rays also slip through hydrogenous materials 100 to 1000 times more easily than neutrons. Norman then dove into the data and discovered that many of these gamma rays had energies well above any normal background noise that may be present during the screening process.

"We believe that under the right conditions, this method could provide the unequivocal signature of gamma radiation indicating that fission has occurred inside the container," Prussin says.

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Already, Prussin and Norman have proven out their method in controlled laboratory experiments at LBNL. The next step is to determine the practicality of the idea by conducting experiments using a full-size container at Livermore that's packed with mock cargo and a sample of depleted uranium.

"I have no idea if we will be able to produce a practical system that can be deployed easily in ports with sufficient sensitivity and high enough throughput to satisfy all our needs," Prussin says. "What I can say is that I think this method has a much higher probability than anything that's been suggested so far. I'm optimistic, but it's no slam dunk."

Stanley Prussin's Home Page

Lawrence Berkeley National Laboratory

Lawrence Livermore National Laboratory Programs for Homeland Security

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