Neutron vision.

A new technology for measuring neutrons might help detect smuggled radioactive materials.

A common type of neutron detector uses a container filled with helium-3, a light isotope that readily reacts with neutrons. When a neutron from a sample of uranium, for example, hits a helium-3 nucleus, a reaction ensues. The helium nucleus breaks and produces one hydrogen nucleus (a proton) and an atom of hydrogen's radioactive isotope, tritium. Usually, these particles are highly energetic and ionize matter. In the presence of a voltage, the ionization produces a small spark, and the reaction is detected.

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Now Charles Clark of the National Institute of Standards and Technology in Gaithersburg, Md., and his collaborators have developed an alternative technique. It promises to detect neutrons over a range of intensities at least 100 times greater than the old method. The new detector simply looks for photons produced when tritium's single electron falls back into a lower-energy state.

A better neutron detector could help prevent nuclear or dirty-bomb terrorism, Clark says. Radioactive materials such as uranium and plutonium emit neutrons, and these are harder to shield than other forms of radiation. Neutrons coming out of a cargo container could then betray the stuff's presence.