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Looking for smuggled nuclear material

Physics News Update #684 also has an article about a program to develop a scanner that can spot as little as 5 kg of highly enriched uranium or 1 kg of plutonium, both shielded, in a cargo shipping container, of which about 6,000,000 enter the US every year.

To address the threat of smuggled nuclear
materials being brought into the U.S., a Lawrence Livermore National
Lab research program is developing a scanner which would examine
cargo shipping containers, which now carry up to 90% of the world's
trade. Six million such containers enter the U.S. each year, the
bulk arriving through 10 ports, the top three being Los Angeles,
Long Beach, and New York-New Jersey. A parcel of radioactive
material, intended as part of a terrorist bomb, would presumably be
shielded inside the cargo container, precluding passive detection.
The Livermore scanner would work in the following way: the
container, on a moving conveyor, would slide past and be exposed to
a neutron beam. The neutrons would irradiate all the contents of
the container, but would especially activate such dangerous
materials such as uranium-235 and plutonium-239. These radioactive
species, perturbed by the neutrons, would fission, resulting in the
emission of characteristic gamma rays detectable in arrays located
downstream of the neutron beam.
Speaking at this week's meeting of the American Physical Society
(APS) in Denver, Thomas Gosnell said that the
goal of the Livermore research is the development of a scanner
capable of locating 5 kg of highly enriched uranium or 1 kg of
plutonium with a false-positive or false-negative rate of 1% or
less. He expects a prototype "nuclear car wash" device would be
working within a year and be deployed on a trial basis in a port,
such as Oakland, California, a year after that.

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