Often called a weapon of mass disruption, not destruction, a dirty bomb -- which uses conventional explosives to spread radioactive material -- causes far fewer casualties than a nuclear explosion. But because such devices are easier to assemble and the ingredients are readily available, government officials and terrorism experts consider a dirty-bomb attack more likely than a terrorist nuclear strike.
"You would need a stick of dynamite and the kind of radioactive source you find in a common smoke detector," said Charles D. Ferguson, co-author of "The Four Faces of Nuclear Terrorism."
There have been several alleged attempts to carry out a dirty-bomb attack.
In June 2002, U.S. authorities arrested Jose Padilla, a former gang member from Brooklyn, on charges of plotting a dirty-bomb strike in the United States on behalf of al Qaeda. Last December, the Department of Energy dispatched scores of nuclear scientists with sophisticated detection equipment to scour several major cities for radiological bombs. In September, British police arrested four men suspected of plotting to set off a dirty bomb in London.
"Any person who could build a car bomb or suicide bomb, like the ones we've seen in Iraq or other places, could couple that to radioactive materials and that is it," Ferguson said.
Such an attack can be carried out by detonating a small conventional bomb that spews the radioactive material and radiation across a small area.
John R. Bolton, undersecretary of state for arms control, said in an interview that the availability of radiological sources presents a significant risk, and that both the United States and the rest of the world "have not paid enough attention to this question. Everybody needs to do more work on that."
Americium, which is found in smoke detectors, is one of eight types of radioactive sources suitable for bombs. Four sources cause external injuries to skin and eyes, and three others, plus americium, can cause extensive internal damage, as well.
Terrorists would need less than a gram of any one of the sources to build a dirty bomb, but the trace amounts found in everyday products are so minuscule that plotters would need more than 1 million smoke detectors to get enough americium for a weapon. Even if a terrorist was able to assemble, plant and detonate a dirty bomb, officials and experts agree the damage would be more psychological than lethal.
"The real effects would be economic shutdown due to contamination, as well as the social and psychological fear created," Ferguson said.