Ninety-two naturally occurring elements make up all substances on Earth. In addition, scientists have made
more than another dozen elements. Each element has unique chemical properties. Moreover, each element comes in different forms, called isotopes,
which differ in their nuclear properties from the original element but have the same chemical properties. All isotopes within an element’s family contain the same number of protons, but have different numbers of neutrons. The number of protons determines the chemical properties, and the combined number of neutrons and protons determines the nuclear properties. In general, isotopes are either stable or unstable. Unstable isotopes are called radioisotopes because they emit radiation and decay to either other unstable or stable isotopes. Radioactive sources are made from radioisotopes. Knowing the type, energy, decay rate, and amount of radiation of particular radioisotopes helps to characterize the security risk posed by a radioactive source.
Ionizing radiation, which has the ability to strip electrons from atoms and break chemical bonds, leading to possible human cell damage, comes in three types:
alpha,
beta, and gamma. Alpha radiation is a stream of alpha particles, each with a helium nucleus consisting of two protons and two neutrons. Beta radiation is the emission of high-speed electrons or their positively charged counterparts (positrons). Gamma radiation
consists of highly energetic light, and differs from alpha and beta radiation in that it is massless and uncharged. It often accompanies the emission of alpha or beta radiation from a particular radioisotope.
The types of ionizing radiation vary in their ability to penetrate materials. A piece of paper can stop most alpha particles. For most beta particles, a thin piece of aluminum or glass suffices to halt them. Blocking gamma radiation, the most penetrating, usually requires thick concrete or lead.
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The penetrating powers of the three types
of ionizing radiation. |
Predicting the rate at which an individual radioisotope decays is impossible because the decay occurs randomly. However, in a large group of identical radioisotopes, the average decay rate can be specified and is usually characterized by the concept of half-life, which is the amount of time required for half of the radioactive sample to decay. After two half-lives, one-fourth of the sample remains; three half-lives, one-eighth; and so on. After seven half-lives, the radioactive substance has decayed to less than one percent of its initial amount. The shorter the half-life, the more frequently the radioactive source emits ionizing radiation.
To visualize the radiation emission, imagine a pipe with a valve connected to a pool of water. The pool represents the source of radiation, and the half-life controls the setting of the valve. A short half-life means the valve is almost fully open; therefore, the pool drains quickly. In contrast, a long half-life means the valve is almost shut, letting out only a trickle of water; thus, the pool empties slowly.
From the security viewpoint, very short and very long half-life radioisotopes present far more limited security risks compared to those having medium-length half-lives on the order of months to decades. In particular, radioactive sources with very short half-lives (hours or minutes or less) do not last long enough (i.e., the pool drains rapidly) to give terrorists sufficient time to produce radiological weapons with those substances; nor do they exist long enough to contaminate places for an appreciable time period. In contrast, those sources with very long half-lives (millions or more years) release radiation at much slower rates (i.e., the pool drains slowly) and typically would not be ideal for radiological weapons devised to maximize the output of radiation during a relatively short time period.