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Union University Department of Physics

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Science Guys > June 2004

June 2004

There is talk that terrorists might use a dirty bomb. What is a dirty bomb and why is it so dangerous?

To us Science Guys, physics is beautiful. From forging advances in medicine to developing principles of communications, physicists have benefited humanity in myriad ways. Yet we are now engaged in a war against terrorists, and terms like dirty bomb are in the popular lexicon. More correctly termed a radiological dispersal device (RDD), a dirty bomb could be used by terrorists to contaminate a city and its inhabitants with lethal radioactive materials. A successful RDD attack could generate panic and economic collapse, and render a city uninhabitable for years.

To understand radioactive materials, we need to understand atomic structure. Elements are composed of particles called atoms. An atom consists of a massive nucleus, or core, and a wispy cloud of electrons buzzing around the nucleus. Electrons are tiny negative electric charges. The nucleus is composed of protons (positive charges) and neutrons (no electrical charge) and has a positive electric charge. While the electron cloud fills much more space than the nucleus, most of an atom's mass is in the nucleus.

Radioactivity originates in the nucleus. The number of protons in the nucleus distinguishes one element from another. An oxygen nucleus has eight protons, whereas a carbon nucleus has six protons. However, a given element may have differing numbers of neutrons. In natural carbon, 98.89% of the atoms possess 6 protons and 6 neutrons, while 1.11% possess 6 protons and 7 neutrons. Elements with different numbers of neutrons are called isotopes of the element. Thus, natural carbon has two different isotopes: carbon-12 and carbon-13.

Some isotopes are unstable (radioactive). Their unstable nuclei spontaneously change to more stable nuclei, emitting particles and energy in a quest to reach a lower energy state. Radioactive isotopes emit pure energy (gamma rays), electrons (beta particles), or alpha particles. Large doses of radiation in humans can induce cancers, cataracts, and birth defects, and in even larger doses is fatal.

A RDD is composed of a conventional explosive laced with radioactive material. When exploded the bomb launches radioactive material as dust into the air. The amount of contamination depends on many factors, such as wind speed and direction. A Union University student recently completed a summer research project sponsored by the Office of Homeland Security, which involved studying wind flow through Oklahoma City. By understanding how winds flow around buildings, we can anticipate how radioactive dust from a dirty bomb will disperse through cities.

Radioactive isotopes can take years, even centuries, to decay into non-radioactive forms, thus rendering the contaminated area uninhabitable for years or until the radioactive isotope is cleaned up, perhaps an impossible task. How would you clean up a hundred pounds of radioactive dust spread over a city?

The physical impact of such a device would certainly rival that of 9/11, but the psychological impact would probably far exceed 9/11 because of public phobias regarding radioactivity.