Radioactivity is the spontaneous disintegration of an unstable atomic nucleus and the emission of particles or electromagnetic radiation. All naturally occurring elements with atomic numbers greater than 83, as well as some isotopes of lighter elements, are radioactive. Three different types of radiation are identified.

Alpha particles (a) are helium nuclei, containing two protons and two neutrons. They are deflected slightly in an electric of magnetic field. Their penetrating power is very low, being stoppable by a thin sheet of aluminum or paper.

Beta particles (b) are electrons capable of travelling at speeds approaching the speed of light. Their low mass allows them to be deflected greatly in an electric or magnetic field, in the opposite direction as the deflection of alpha particles. Their high speed gives them greater penetrating power than alpha particles. Some beta particles can penetrate several centimetres of aluminum. (Some refer to beta particles as "beta negative particles", to distinguish them from beta positive particles -- positrons.) Alpha particle emissions and beta particle emissions change the composition of the nucleus.

Gamma rays (g) are high energy electromagnetic radiation with short wavelengths. Gamma rays, unlike alpha and beta particles, do not change the composition of the nuclide. They have the highest penetrating power, being able to penetrate at least 30 centimetres of lead.

Background radiation comes from a variety of radioactive sources. Cosmic rays penetrating the Earth's atmosphere from outer space usually account for less than 25% of background radiation (but this depends on altitude). Minute quantities of naturally occurring radioactive sources in the surroundings (e.g., soil, air, drinking water, building materials, food, etc.) also contribute to background radiation.

Nuclides radiations affect the emulsion of photographic film, ionize surrounding air molecules, make certain compounds fluoresce, and have certain special biological effects. They undergo radioactive decay. Nuclide charts, with atomic number plotted against neutron number, are used in nuclear physics to illustrate a decay series.

Radioactivity is found in naturally occurring sources and in artificially produced ones. People are constantly being exposed to radiation from a variety of natural and human-created sources. Exposure should be minimized, but it can never be reduced to zero. Dosimetry is the measurement of radiation and the study of its effects on living organisms.

There are several different units used to measure radiation.

The absorbed dose describes the amount of energy deposited per kilogram of exposure time, measured in the gray (Gy).

1 Gy = 1 J / kg = 100 rads (rads are non-SI, but in general use.)

The biological damage produced on a given organism is called the dose equivalent, measured in sieverts (Sv).

1 Sv = 100 rem = 10⁵ mrem

Becquerel (Bq) is the activity of a source produced when one disintegration per second occurs from a radioactive source.

1 Bq = 1 disintegration per second

kBq and MBq are often used to express the radioactivity of a source.

1 curie (Ci) = 3.7 × 10¹⁰ Bq