Man has always been exposed to natural radiation arising from the earth as well as from outside the earth. The radiation we receive from outer space is called cosmic radiation or cosmic rays. Fallout from nuclear explosives testing, and small quantities of radioactive materials released to the environment from coal and nuclear power plants, are also sources of radiation exposure to man. Radioactivity is the term used to describe disintegration of atoms. The atom can be characterized by the number of protons in the nucleus.
Some natural elements are unstable.
Therefore, their nuclei disintegrate or decay, thus releasing energy in the form of radiation. This physical phenomenon is called radioactivity and the radioactive atoms are called nuclei. The radioactive decay is expressed in units called becquerels. One becquerel equals one disintegration per second. The radionuclides decay at a characteristic rate that remains constant regardless of external influences, such as temperature or pressure.