In October, 2005, a radiation sensor at the Port of Colombo, in Sri Lanka, signalled that the contents of an outbound shipping container included radioactive material. The port's surveillance system, installed with funds from the National Nuclear Security Administration, an agency within the Department of Energy, wasn't yet in place, so the container was loaded and sent to sea before it could be identified. After American and Sri Lankan inspectors hurriedly checked camera images at the port, they concluded that the suspect crate might be on any one of five ships-two of which were steaming toward New York...

The Bush Administration is now spending about four hundred million dollars annually on radiation-detector research, but nuclear physicists who have studied the technology disagree about how discriminating these sensors might become. One point on which everyone agrees, however, is that, of all the potentially dangerous radioactive isotopes, it will always be most difficult to detect highly enriched uranium-235, one of the two materials, along with plutonium, used to make fission weapons...Radiation sensors, then, will always be more effective against a Dhiren Barot than against, say, the Pakistani nuclear scientist Abdul Qadeer Khan, a metallurgist who has spent many years studying fission weapons and highly enriched uranium, as well as the challenges of international smuggling...

Jeffrey Lewis, a nonproliferation specialist at the New America Foundation, said that radiation sensors had probably attracted support within the Bush Administration because they appeal to the instincts of defense thinkers who want to act boldly in the world but are also, at heart, isolationists. "You don't have to go mess with the difficult diplomacy of getting the Pakistanis to secure their material if you can ring the country with interceptors, or ring the country with detectors," he said. "Even if it's ineffective, it's something that we can do entirely ourselves-that's just really appealing to these guys..."

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