diet. Some of the ingested potassium is absorbed in tissue. That process is not limited to potassium, but can occur with iodine, sodium, radium, and so on. Therefore, all persons contain some radioactivity.

Each radioactive isotope has unique properties. One property is the type of particles emitted, and another is the energy of the particles emitted. No two radioactive isotopes emit the same combination of particles and energies. Therefore, one can identify the presence of a specific isotope at a given location by measuring the types and energies of the emitted particles.

Radioactive decay is a random process: it is impossible to determine when a given nucleus will decay. However, it is possible to estimate how many nuclei in a group will decay during a given period. The half-life of an isotope is the time it takes for half the nuclei in a group or sample to decay. Thus, isotopes with short half-lives decay rapidly and those with long half-lives decay more slowly. No two isotopes have the same half-life. For example, the half-life of nitrogen-16 (16N) is 7.3 seconds; that of radon 222 (222Rn), 3.8 days; that of 131I, 8 days; and that of uranium-238 (238U), 4.5 billion years.

Radioactivity specifically refers to the rate at which decays occur. The amount of radioactivity present depends on the number of radioactive atoms and their corresponding half-life. The rate at which atoms are decaying is proportional to the number of atoms divided by the half-life. This decay rate is described in units of either Becquerels (Bq) in the International System, SI, of units or Curies (Ci) in the traditional system of units used in the United States; 1 Bq is equal to 1 decay per second, and 1 Ci is equal to 37 billion decays per second. The amount of radioactivity is often stated in terms of a millicurie (mCi), which is one thousand times smaller than a Curie. One microcurie (µCi) is one million times smaller than a Curie and one picocurie is one trillion times smaller than a Curie. The amount of radioactivity at any time is reduced by one-half in a period of time equal to one half-life.

Radioactivity generates radiation by emitting particles. Radioactive materials outside the body are called external emitters, and radioactive materials located within the body are called internal emitters.

Types of Ionizing Radiations

Radioactive nuclei can emit several kinds of particles, but there are three primary types: alpha particles (α), beta particles (β), and photons that are either x rays or gamma rays (γ). Several properties distinguish those particles from one another. One is electric charge; alpha particles are emitted with a positive charge of 2, beta particles are emitted with either 1 negative charge (electron) or 1 positive charge (positron), and x rays and gamma rays have no charge and are thus neutral.

Another important property is penetration of the particles through matter. Alpha particles lose energy rapidly and stop in a very short distance. Most travel



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