More accurately, a dirty bomb, called a radiological dispersal device (RDD), can be used to contaminate a city and its inhabitants with deadly radioactive materials

More accurately, a dirty bomb, called a radiological dispersal device (RDD), can be used to contaminate a city and its inhabitants with deadly radioactive materials. A successful RDD attack can cause panic and economic collapse, rendering a city uninhabitable for years.

To understand radioactive materials, we need to understand the structure of the atom. Elements are made up of particles called atoms. An atom is made up of a massive nucleus, or core, and a thin cloud of electrons that orbit the nucleus. Electrons are tiny negative electric charges. The nucleus is made up of protons (positively charged) and neutrons (uncharged) and has a positive electric charge. While the electron cloud fills much more space than the nucleus, most of the mass of the atom is located in the nucleus.

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

Some isotopes are unstable (radioactive). Their unstable nuclei spontaneously decay into more stable nuclei, emitting particles and energy in an attempt to reach a lower energy state. Radioactive isotopes emit net energy (gamma rays), electrons (beta particles), or alpha particles. High doses of radiation in humans can cause cancer, cataracts, and birth defects, and are fatal at even higher doses.

An RDD consists of a conventional explosive impregnated with radioactive material. When detonated, the bomb throws radioactive material into the air as dust. The extent of contamination depends on many factors, such as wind speed and direction.

The nature of radioactivity is such that not all elements are suitable for use in an RDD. The half-life of radioactive elements (the time it takes for half the atoms in a given mass of radioactive isotopes to decay into a stable element) ranges up to 700 million years in the case of uranium-235, which is not regarded as highly radioactive. Other isotopes have half-lives of a few hours, making them exceedingly dangerous for a short period—too short for terrorists to construct a bomb and explode it. The danger of radioactivity from such an isotope could be over within a short period. The half-life of cobalt-60, on the other hand, is 5.3 years, making it an ideal element for use in an RDD.

While acquiring a sufficient quantity of a suitable element is not simple, . Radioactive isotopes are widely used, albeit in small quantities (home smoke detectors, for example, use minute quantities of radioactive materials).

Two isotopes whose availability is of particular concern are cesium-137, which is used in radiation therapy devices for cancer treatment and oil well monitoring, and cobalt-60, which is also used in cancer treatment and industrial radiography. These isotopes are used in devices that typically operate under little security and are sometimes abandoned (or "orphaned"). These isotopes can be moved relatively easily, as they are most harmful only if inhaled. There are numerous sources of such elements in legitimate trade, or they may be stolen during transport. Such isotopes, if collected in sufficient quantities, are considered "ideal" elements for use in RDD.