The problem of detecting smuggled nuclear weapons or devices presents an enormous challenge for security officials. More than 6 million containers enter U.S. ports annually. West Coast facilities alone process about 11,000 a day, or an average of eight every minute. A single container can hold up to 57,000 pounds.

Officials have been attempting to figure out how to inspect containers for smuggled nuclear materials without disrupting the flow of the nation's commerce.

The physicists, from Lawrence Livermore National Laboratory in Berkeley, Calif., think they have come up with the solution.

"We have identified a new radiation signature, unique to fissionable material, that exploits high-energy, fission-product, beta-delayed, gamma ray emissions," said lead researcher Thomas Gosnell. "Fortunately, this signature is robust in that it is very distinct compared to normal background radiation, where there are no comparable high-energy gamma rays."

In other words, using the right type of detector, inspectors would be able to see through containerized cargo and determine if any of it contains illicit nuclear material. Hence, terrorists would not be able to bury their ill-gotten booty in a crowd.

Gosnell and colleagues presented their findings at the American Physical Society's annual meeting.

The detector actually works by first aiming a neutron generator -- either buried in the ground or ringing a portal in sort of a "nuclear carwash" -- at the cargo, Gosnell explained. If the neutrons produced by the generator hit any nuclear materials, they will cause gamma-radiation-producing fission that can be picked up by the detector.

Even though nuclear fission is the process by which reactors and bombs are produced, Gosnell said there is no danger from the device. He said his team found "the (radiation) levels of the cargo, even under the worst-case assumptions, are low enough for the cargo to be considered non-radioactive for shipping by the Department of Transportation."

One possible problem with the neutron generator involves its use on containers filled with food, which compose about one-third of the material entering U.S. ports. Gosnell said presently there are no applicable standards for irradiation of food imports.

"All food is very slightly radioactive, mostly from potassium," he explained. "We want to make sure that if we activate food, it doesn't get very radioactive. There are no standards on this, it turns out."

Gosnell said government regulators told him there would be no problem as long as the device did not add more radioactivity to food than already exists.

"That's probably a good goal to shoot for," he said.

Gosnell predicted an operating detector would be ready to test in place at a cooperative port within two or three years.

Meanwhile, the United States has not made much progress dealing with radiation hazards from so-called "dirty bombs," said Jaime Yassif of the Federation of American Scientists.

Also speaking at the APS meeting, Yassif said a radiological weapon "would not kill large numbers of people. It is primarily a weapon of economic and psychological disruption."

Jose Padilla was arrested last year for plotting to explode a dirty bomb. Such a bomb -- formally called a radiological dispersion device by arms experts -- is a conventional explosive layered with highly radioactive material. When the bomb explodes, it can spread radiation over a large area.

In the aftermath of such an attack, people would be unable to enter a large area of a city until it was decontaminated.

"Daily business activity would halt, and real estate values would plummet," Yassif said. "Segments of the city might have to be abandoned or demolished if they could not be decontaminated to publicly acceptable levels."

Yassif said the United States must try to prevent a dirty bomb attack, but if such efforts failed, the nation should establish decontamination procedures that could be implemented as quickly as possible.

"Getting an early start is crucial for two reasons," she explained. "One, radiological decontamination can become more difficult as time passes and, two, rapidly restoring urban activity, when possible, will drive down the economic and psychological costs associated with an attack."

Peter Zimmerman, professor of science and security at Kings College in London, said he disagreed with Yassif on only one point.

"A dirty bomb is not necessarily high explosives and nuclear isotopes, radioactive isotopes," Zimmerman said. "There are enormously better ways to spread the radiation around. ... It is very simple to take very tiny radioactive sources of, let's say, cesium 137, and glue them under a table ... where people are going to sit for half an hour. Later they may have absorbed 100 rad or 150 rad."

A dirty bomb attack would be an economic disaster of massive proportions, Zimmerman said, and noted insurance policies in the United States do not cover radiation damage.

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