Driven by the fear that terrorists will attempt to smuggle components for a radiological dispersal device, or dirty bomb, into the country, U.S. Customs and Border Protection announced plans two years ago to deploy detectors to check everyone arriving in the U.S. for radiation sources.

Many sources, however, were already here.

Radioactive sources — usually sealed in cylindrical, stainless-steel containers roughly the size of a roll of quarters — have been used in medicine and industry since 1954. In oil fields, for example, radioactive sources provide a well-logging technique for determining mineral composition through neutron absorption.

No one seems to know exactly how many radioactive sources there are or where many of them might be, although the Nuclear Regulatory Commission once estimated there were over 27,000 of them. There is no tracking mechanism.

"There is a reasonable risk that a dirty bomb attack will occur because it is not difficult to obtain radioactive materials, and assembling a dirty bomb is fairly simple," said radiation safety officer P. Andrew Karam, Rochester Institute of Technology.

A dirty bomb is nothing more than an ordinary, conventional high-explosive device used to scatter radioactive source material, said Warren Campbell of the Maryland Fire and Rescue Institute.

Some experts believe the threat posed by dirty bombs is not that serious if risk is measured in terms of deaths or illnesses.

“It is extremely unlikely that a dirty bomb would kill anyone — beyond those killed by the high explosive — or that anyone would become ill or incur significantly increased chances of developing cancer as a result of such an attack,” said physicist Steve Fetter, University of Maryland’s School of Public Affairs.

The principal RDD product is not carnage, it’s terror. Even though unlikely to produce heavy casualties, detonation of a dirty bomb in the heart of any metropolis could shut down the city center for weeks, with fear reverberating through the country that no population center is safe any longer, Fetter said.

Detection technologies for first responders continue to evolve.

"First responders need direct-read instrumentation that doesn’t require interpretation," said Eric Lamar, an adviser to International Association of Fire Fighters President Harold Shaitberger.

If the detection instrument goes into alarm, Lamar said, it must accurately detect a threshold level that will then trigger basic actions to be taken to reduce risk.

"Sophisticated instrumentation that requires constant re calibration or that isn’t engineered for rough handling consistently fail in the emergency response arena," he said.

Instruments abound that can detect radioactive materials, ranging from simple Geiger counters to sophisticated multi channel analyzers and gamma spectroscopy equipment.

U.S. Customs and Border Protection deploys a range of technologies in the nation's ports to scan incoming shipments, some designed specifically to detect nuclear/radioactive materials. At some ports, CBP is installing portal radiation detectors, which are fixed devices that can detect various types of radiation emanating from shipping containers.

In addition, isotope identifiers exist that allow inspectors to determine whether a source of radiation is a possible terrorist threat or a harmless commercial-medical source of radiation.

CBP is also exploring the use of crane mounted radiation detection devices to detect radioactive materials in cargo containers.

Other detection technologies in use include gamma-ray imaging systems, density-measuring devices, fiber optic scopes, acoustic inspection systems and low tech tools.

Many of these detection systems suffer from the same malady, however.

"One of the biggest drawbacks is that any radiation source can be lead-shielded to make detection difficult or impossible," Karam said.

Because lead shielding is heavy and bulky, detecting the shield itself is one way to find radioactive materials. Los Alamos National Laboratory has found a promising way of detecting lead without opening every shipping container.

Unlike X-ray imaging, which depends on small doses of artificial radiation to create images of dense objects such as bones or smuggled metal objects, Los Alamos researchers use muon radiography, which needs no additional radiation.

Muons, created naturally when cosmic rays from space interact with the Earth's atmosphere, pass through the Earth's surface at a rate of approximately 10,000 particles per square meter per minute.

Placing an array of sensors above and below a target, the LANL scientists developed a computer algorithm to detect variations in the muons' paths before and after striking a heavy metal target. The change in direction by the muons was the key to the detection technology, permitting the researchers to build three-dimensional images of the target objects.