The nucleus of each atom has a specific number of protons and neutrons and is either stable or unstable, depending on the relative number of each. The most stable atoms are those that have an equal number of protons and neutrons. Atoms that are unstable are radioactive. An atom that is radioactive can also be called a radionuclide. Of the known nuclides (approximately 2,000), only 264 are stable, and of the known radionuclides (approximately 1,700), only 70 occur in nature. The rest are man-made. Unstable atoms undergo a process called radioactive decay to reach a more stable state.

While a radionuclide is going through the process of decay, energy is released from the atom in one of three modes: alpha, beta, or gamma radiation. These modes may take several steps, involving only the nucleus or the entire atom. Each radionuclide has one or more characteristic modes of decay. The particular...