"It was just sittin' in a five-gallon bucket," says the shop's owner in a syrupy drawl. Police determined the device used by repair crews to check for cracks in pipe welds had been stolen from a pipeline-company truck six months earlier in nearby Mobile.

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But the unusual thing about this story isn't that the device was stolen. It's that it was found.

That's because roughly 2 million small-but-valuable radioactive contraptions are used in the US in everything from construction to healthcare to scientific research. And every year, hundreds of them are lost, stolen, even abandoned. Most are never retrieved, and 30,000 are unaccounted for, according to some estimates.

In the post-Sept. 11 era, that's giving experts cause for concern: If these devices can turn up at an Alabama pawnshop, they could just as easily be hoarded by terrorists to create "dirty bombs" conventional explosives laced with radioactive material.

"If you were going around snatching these smaller devices over a period of years and putting them all in a truck bomb, it could be as powerful as a bomb with a single, big radiation source," says Edwin Lyman, of the Nuclear Control Institute in Washington.

Clearly, not all small radioactive items would work as dirty-bomb ammunition the radiation many emit is extremely weak. Still, terrorists could create a dangerous weapon by combining several dozen minor sources with a simple explosive, says Friedrich Steinhausler, a Stanford University nuclear physicist.

The damage from a "dirty bomb" would depend on many things, including the strength of the explosive, the amount of radioactive material, and how far winds would spread the toxic particles. Experts say such bombs could cause fatalities in the immediate area of detonation and a range of health complications in a wider area. Their real insidiousness would be in the low or moderate levels of radiation spread, possibly requiring whole sections of a city to be abandoned for years.

That's because radiation cleanup is, at best, expensive and difficult sometimes impossible. In all, rather than being a "weapon of mass destruction," a "dirty bomb" is more like a "weapon of mass disturbance," says Dr. Steinhausler.

Authorities respond

The threat has rattled federal regulators. The Nuclear Regulatory Commission (NRC) is reviewing how the devices are monitored.

"We're looking at requiring licensees to increase security," says John Hickey, chief of the NRC division that oversees the devices.

But new measures might only include better locks and stronger storage facilities, and some critics worry that isn't enough. In general, they fault the NRC for overlooking the smaller radiation devices and focusing instead on safety at higher-profile nuclear plants.

The devices in question include practically harmless emergency-exit signs that rely on radioactive isotope for power rather than electricity, which can fail. If broken open, these could expose a person to radiation less intense than a dentist's X-ray. By contrast, the pencil-sized rods used to irradiate food are so dangerous that direct exposure could be quickly fatal, say experts. (This also makes stealing them very difficult.)

As for the pawnshop item, it contains a piece of iridium-192 that's smaller than a pea. The iridium is shielded by depleted uranium to keep radiation from escaping. To use the device, crews put the radiation source on one side of a pipe and a special film on the other. The radiation creates an image that shows hidden cracks or other weaknesses.

Experts say if a person stood within one foot of that unshielded iridium nugget for two hours, they could receive a fatal radiation dose.

The sheer number of such devices in use in the US makes detailed tracking tough and mishaps common.

On March 15, a $6,000 radioactive moisture-density gauge used to determine if fresh concrete has fully dried was reported stolen from a Maryland construction site. Such devices typically contain several grams of cesium-137, a highly radioactive material especially dangerous because it can persist in the environment for centuries and can work its way into the food chain. These devices more than exit signs, for instance are what worry authorities. They contain enough material to be dangerous, especially if combined with other similar sources. Yet they aren't so toxic like food-irradiation rods as to require complicated equipment or advanced knowledge to handle.

In February, a Wisconsin paper manufacturer discovered it had mistakenly shipped a radioactive device used to measure paper-pulp density to China, according to NRC documents. An executive scrambled and arrived ahead of the shipment. But when he was at lunch, the shipment's containers were unloaded, and only a frantic search led to the device. Apparently the initial confusion in Wisconsin was caused because the device was coated in paper mulch that obscured warning labels.

In 1998, 19 vials of cesium-137 disappeared from a Greensboro, N.C., hospital, where they're used in medical treatments. Because it happened around the time of the Final Four basketball tournament, concern about terrorism was high. So the federal government tasked its Nuclear Emergency Search Team with finding the vials, which were never retrieved.

In 1996, two stolen industrial cameras were sold to scrap-metal dealers in Houston. After one was broken open, 11 adults and two children were exposed to high radiation levels that experts say significantly boosted the risk of later medical complications.

Converting items like these into "dirty bombs" is a real possibility mostly because it requires only simple scientific knowledge, say experts. "It's high school science, not rocket science," says Stanford's Steinhausler.

But easier to track

One small upside of the radioactive materials in commercial use is that they're typically easier to detect than more-potent elements like plutonium. When covered with simple tinfoil, plutonium is nearly impossible to detect, for instance. But even when shielded by lead, cesium-137 can be tracked by sensitive detectors.

Finally, much of the problem comes down to economics. For instance, it currently costs about $400 per cubic foot to dispose of materials like cesium-137, says Lyudmila Zaitseva of Stanford's Institute for International Studies. That's roughly 10 times the amount of fines for improper disposal of the material. The lack of high fines, she says, can lead to shoddy tracking even deliberate abandonment.

Separately, Dr. Lyman observes that boosting the security of these devices would add to already high costs in construction, healthcare, and other fields. Whether it's worth it, he says, "is a tough societal question."