Radioactivity

Nuclei undergoing nuclear decay release various kinds of emissions. We will discuss three of these emissions: alpha particles, beta particles, and gamma rays.

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A D V E R T I S E M E N T

All three are forms of ionizing radiation, so called because their passage through matter leaves a trail of ions and molecular debris.

Alpha particles

An alpha particle is identical to a helium atom that has been stripped of its two electrons; thus, an alpha particle contains two protons and two neutrons. Because an alpha particle has no electrons to balance the positive charge of the two protons, it has a charge of +2 and can be represented as He2+. If a particle has a charge, whether negative or positive, it can be shown as a superscript. Thus He2+ means a helium atom that has lost two electrons and has a +2 charge. The symbol O2- means an oxygen atom that has added two electrons and thus has a charge of -2. Atoms that have acquired a charge by losing or gaining electrons are called ions. Besides He2+, other symbols for this particle are

When ejected from a decaying nucleus, alpha particles interact with all matter in their path, whether it be photographic film, lead shielding, or body tissue, stripping electrons from other atoms as they go. In their wake, they leave a trail of positive ions (atoms from which electrons have been removed) and free electrons. A single alpha particle, ejected at high speed from a nucleus, can create up to 100,000 ions along its path before it gains two electrons to become a neutral helium atom. In air, an alpha particle travels about 4 cm before gaining the two electrons. Within body tissue, its average path is only a few thousandths of a centimeter. An alpha particle is unable to penetrate the outer layer of human skin. Because of this limited penetrating power, external exposure to alpha particles is not nearly as serious as internal exposure. If a source of alpha emissions is taken internally, the alpha radiation can do massive damage to the surrounding tissue; therefore alpha emitters are never used in nuclear medicine.