There are a number of materials terrorists can acquire to construct a "dirty bomb." These materials may be stolen, purchased on the black market or purchased legally. Such materials are commercially available within the medical and agricultural industries, in oil drilling equipment and even in very small amounts in household smoke detectors.
Cesium-137, Cobalt-60, and Americium-241 are considered to be the most likely materials for use in a dirty bomb due to their availability and their relative ease of handling. While uranium and plutonium are much stronger and more lethal agents, they are also the significantly more difficult to acquire and control.
Cesium-137 is a talc-like metallic powder found in cancer treatment radiation machines. It is dangerous even in small amounts and according to the U.S. Department of Homeland Security, Cesium-137 has a half-life of 33 years.
For more information on Cesium-137 and its properties and effects, go here: U.S. Centers for Disease Control and Prevention
A solid metal that is commercially produced for use in medical radiation therapy, food irradiation and linear accelerators. Its half-life is approximately five years, according to the U.S. Agency for Toxic Substances and Disease Registry. The cobalt used in food irradiation takes the form of small, radioactive "pencils." They are shipped in special hardened steel canisters that have been designed and tested to survive crashes without breaking.
For more information on Cobalt-60 and its properties and effects go here: U.S. Centers for Disease Control and Prevention
This is a man-made radioactive agent that is used in oil drilling and surveying equipment and in very small amounts in household smoke detectors. It has a half-life of 432 years, according to the U.S. Agency for Toxic Substances and Disease Registry.
For more information on Americium 241 and its properties and effects go here: U.S. Agency for Toxic Substances and Disease Registry