We are exposed to nuclear radiation every day. Natural sources include cosmic radiation, radiation from lighter, unstable nuclei produced by the bombardment of the atmosphere by cosmic radiation, and radiation from heavy, unstable nuclei produced by the decay of long-lived nuclides in the earth's crust. Artificial sources include medical procedures, commercial products, and fallout from nuclear testing.
Nuclear radiation can cause biological damage because it is highly energetic.
Nuclear radiation loses its energy when it passes through matter by ionizing the absorbing material. For this reason, nuclear radiation is called ionizing radiation. In the ionization process, neutral atoms in the absorbing material lose electrons, forming positive ions. Frequently, the ejected electrons possess sufficient energy to cause other atoms to ionize. The magnitude of the radioactivity in a sample can be expressed as an activity, exposure, or absorbed dose.