THE UNTHINKABLE.

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.

Sri Lanka is a locus of guerrilla war and arms smuggling. It is not far from Pakistan, which possesses nuclear arms, is a haven for Al Qaeda, and has a poor record of nuclear security. The radiation-emitting container presented at least the theoretical danger of a "pariah ship," Vayl Oxford, the director of the Domestic Nuclear Detection Office, which is part of the Department of Homeland Security, said. It seemed plausible, if unlikely, that Al Qaeda or rogue Pakistani generals might load a bomb onto a cargo vessel. Within days, American satellites located the five suspect ships and intelligence analysts scrutinized their manifests; a team at the National Security Council took charge. One ship, it learned, was bound for Canada, and another for Hamburg, Germany. The White House decided to call in its atomic-bomb squad, known as NEST, the Nuclear Emergency Support Team--scientists who are trained to search for nuclear weapons. One team flew to Canada and a second to Europe, where it intercepted one of the ships at sea before it could reach Hamburg. They found nothing.

The United States Coast Guard stopped the two New York-bound ships in territorial waters, about ten miles offshore; from that distance, if there was a nuclear weapon on board a detonation would cause relatively little harm. Scientists boarded the vessels, shouldering diagnostic equipment, but these ships, too, turned out to be clean; as it happened, the offending vessel was on an Asian route, and its cargo was scrap metal mixed with radioactive materials that had been dumped improperly. The entire episode, which was not disclosed to the public, lasted about two weeks.

This sometimes nerve-racking exercise resulted in no more than the disposal of some radioactive waste. It was also the first major defensive maneuver triggered by a shield that the United States is attempting to build as a defense against a clandestine nuclear attack. The idea, in essence, is to envelop the country in rings of radiation detectors and connect these sensors to military and police command centers, which would then respond to unexplained movements of nuclear material. The project, comparable in ambition to ballistic-missile defense, is the first of its kind in the atomic age. The plan has already attracted criticism from some scientists and defense strategists, primarily because, as with missile defense, the project promises to be expensive and would require leaps of ingenuity to overcome technical problems presented by the laws of physics.

Still, with little public discussion this "layered defense," as it is described by its proponents, is being deployed. The federal government has distributed more than fifteen hundred radiation detectors to overseas ports and border crossings, as well as to America's northern and southern borders, domestic seaports, Coast Guard ships, airports, railways, mail facilities, and even some highway truck stops. More detectors are being distributed each month. NEST and the Federal Bureau of Investigation maintain a permanent team to respond to events in Washington and along the Northeast Corridor; a second team trained to dismantle nuclear weapons is based in Albuquerque, and eight other teams able to diagnose radioactive materials operate on continuous alert elsewhere in the country. Since the terrorist attacks of September 11, 2001, NEST teams have been deployed about twice a year because of specific threats reported by intelligence agencies, including at least two instances, apart from the Sri Lankan episode, where they boarded a ship approaching the United States. NEST units also discreetly screen vehicles, buildings, and people at designated events such as political conventions and the recent N.B.A. All-Star Game, in Las Vegas. In the United States alone, the sensors generate more than a thousand radiation alarms on an average day, all of which must be investigated.

The world, it turns out, is awash in uncontrolled radioactive materials. Most are harmless, but a few are dangerous, and many detectors are still too crude to distinguish among different types of radiation; they ring just as loudly if they locate nuclear-bomb material or contaminated steel or, for that matter, bananas, which emit radiation from the isotope potassium-40. So far, the result has been a cacophony of false alarms, which, in most cases, are caused by naturally occurring radiation that has found its way from soil or rock into manufactured products such as ceramic tiles. In addition, people who have recently received medical treatments with radioactive isotopes such as thorium can set off the detectors. At baseball's All-Star Game in Detroit in 2005, unobserved NEST scientists screened tens of thousands of fans entering the stadium, and their sensors rang just once--reacting to the former Secretary of Energy Spencer Abraham, who was radioactive from a recent doctor's visit.

Detritus from nuclear commerce that has slipped through American and international regulatory systems is another periodic source of alarms, and one that has proved to be a greater cause of concern. Virtually none of the loose material detected so far would be useful to a terrorist seeking to build a fission weapon--a bomb of the sort that was dropped on Hiroshima. A disquieting fraction of it, however, might be useful for what the American defense bureaucracy calls a "radioactive dispersal device," more commonly known as a dirty bomb. There is recent evidence, too, that Al Qaeda-inspired radicals are pursuing such a weapon.