Radioactivity

Radioactive Materials

Most atoms found in nature are stable and never change, unless made to do so by an outside force. However, some atoms are unstable; these we call radioactive. Radioactive atoms undergo a change called disintegration. This disintegration does not mean that the atom falls apart, but that a particle with mass and charge is emitted from or captured by the nucleus of the atom, often accompanied by electromagnetic (x-ray and/or gamma) radiation. The remaining material in the nucleus of the atom rearranges itself and becomes the nucleus of either a different element or of another isotope of the same element.

Quantity of Radioactive Material/Activity

The unit for the quantity of radioactive material is based on the disintegration rate of the particular material. The basic unit is the curie, which is the quantity of radioactive material in which 37 billion atoms disintegrate per second. The usual amounts used in medicine are the millicurie (mCi) and the microcurie (uCi). The millicurie is one-thousandth of a curie, and the microcurie is one-millionth of a curie, or in other words, 37 million disintegrations per second and 37 thousand disintegrations per second, respectively.

Radioactive Decay/Half-Life

The term decay is used when referring to radioactive disintegration, and the term half-life conveniently expresses the length of time that it takes for any radioisotope to decay to half of its original level of activity. In other words, half of the original radioactive atoms have changed to other isotopes or elements. The half-life for a given isotope is the same, is constant, regardless of the original number of atoms. In one half-life, the number of radioactive atoms drops to one-half of the original number, and in two half-lives, the number of radioactive atoms is now one-half of one-half, or one-quarter.

Types and Energies of Radiation

During the process of radioactive decay, an atom may emit various forms of radiation. For medical purposes, only four types of radiation are significant: alpha particles, beta particles, x-rays, and gamma rays. In summary, alpha particles are helium nuclei (helium atoms with the electrons removed), beta particles are electrons, and x-ray and gamma rays are photons (electromagnetic radiation).

The energies of the radiations, as well as their ability to penetrate matter, vary a great deal. The range of beta particles in tissue varies from a fraction of a millimeter to more than one centimeter. This depends on the energy of the source of radiation and the distance of the source from the skin surface. Many particles (e.g., alphas and low energy betas) are absorbed in a few centimeters of air.

X-rays and gamma rays are very penetrating. Some will pass completely through the human body and produce no effect. Only rays stopped by the body produce changes in tissue or leave shadows on x-ray film. There is no essential difference between x-rays and gamma rays; both cause the same effects, and both are detected and measured in the same way.

Medical Radioisotopes

While the quantities of radioactive materials used for diagnostic studies are relatively small with correspondingly small exposures to personnel, the amounts used for therapy can be large with high exposures near the patient. Therefore, precautionary measures must be taken to maintain doses As Low As Reasonably Achievable (ALARA).

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