Definition of Radioactivity

Radioactivity symbol
This is the international symbol for radioactivity. Caspar Benson / Getty Images

Radioactivity is the spontaneous emission of radiation in the form of particles or high energy photons resulting from a nuclear reaction. It is also known as radioactive decay, nuclear decay, nuclear disintegration, or radioactive disintegration. While there are many forms of electromagnetic radiation, they are not always produced by radioactivity. For example, a light bulb may emit radiation in the forms of heat and light, yet it is not radioactive. A substance that contains unstable atomic nuclei is considered to be radioactive.

Radioactive decay is a random or stochastic process that occurs at the level of individual atoms. While it is impossible to predict exactly when a single unstable nucleus will decay, the rate of decay of a group of atoms may be predicted based on decay constants or half-lives. A half-life is the time required for half of the sample of matter to undergo radioactive decay.

Key Takeaways: Definition of Radioactivity

  • Radioactivity is the process by which an unstable atomic nucleus loses energy by emitting radiation.
  • While radioactivity results in the release of radiation, not all radiation is produced by radioactive material.
  • The SI unit of radioactivity is the becquerel (Bq). Other units include the curie, gray, and sievert.
  • Alpha, beta, and gamma decay are three common processes through which radioactive materials lose energy.


The International System of Units (SI) uses the becquerel (Bq) as the standard unit of radioactivity. The unit is named in honor of the discoverer of radioactivity, French scientists Henri Becquerel. One becquerel is defined to be one decay or disintegration per second.

The curie (Ci) is another common unit of radioactivity. It is defined as 3.7 x 1010 disintegrations per second. One curie equals 3.7 x 1010 bequerels.

Ionizing radiation is often expressed in units of grays (Gy) or sieverts (Sv). A gray is the absorption of one joule of radiation energy per kilogram of massA sievert is the quantity of radiation associated with a 5.5% change of cancer eventually developing as a result of exposure.

Types of Radioactive Decay

The first three types of radioactive decay to be discovered were alpha, beta, and gamma decay. These modes of decay were named by their ability to penetrate matter. Alpha decay penetrates the shortest distance, while gamma decay penetrates the greatest distance. Eventually, the processes involved in alpha, beta, and gamma decay were better understood and additional types of decay were discovered.

Decay modes include (A is atomic mass or number of protons plus neutrons, Z is atomic number or number of protons):

  • Alpha decay: An alpha particle (A =4, Z=2) is emitted from the nucleus, resulting in a daughter nucleus (A -4, Z - 2).
  • Proton emission: The parent nucleus emits a proton, resulting in a daughter nucleus (A -1, Z - 1).
  • Neutron emission: The parent nucleus ejects a neutron, resulting in a daughter nucleus (A - 1, Z).
  • Spontaneous fission: An unstable nucleus disintegrates into two or more small nuclei.
  • Beta minus (β−) decay: A nucleus emits an electron and electron antineutrino to yield a daughter with A, Z + 1.
  • Beta plus (β+) decay: A nucleus emits a positron and electron neutrino to yield a daughter with A, Z - 1.
  • Electron capture: A nucleus captures an electron and emits a neutrino, resulting in a daughter that is unstable and excited.
  • Isomeric transition (IT): An excited nucleus releases a gamma ray resulting in a daughter with the same atomic mass and atomic number (A, Z),

Gamma decay typically occurs following another form of decay, such as alpha or beta decay. When a nucleus is left in an excited state it may release a gamma ray photon in order for the atom to return to a lower and more stable energy state.


  • L'Annunziata, Michael F. (2007). Radioactivity: Introduction and History. Amsterdam, Netherlands: Elsevier Science. ISBN 9780080548883.
  • Loveland, W.; Morrissey, D.; Seaborg, G.T. (2006). Modern Nuclear Chemistry. Wiley-Interscience. ISBN 978-0-471-11532-8.
  • Martin, B.R. (2011). Nuclear and Particle Physics: An Introduction (2nd ed.). John Wiley & Sons. ISBN 978-1-1199-6511-4.
  • Soddy, Frederick (1913). "The Radio Elements and the Periodic Law." Chem. News. Nr. 107, pp. 97–99.
  • Stabin, Michael G. (2007). Radiation Protection and Dosimetry: An Introduction to Health Physics. Springer. doi:10.1007/978-0-387-49983-3 ISBN 978-0-387-49982-6.
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Helmenstine, Anne Marie, Ph.D. "Definition of Radioactivity." ThoughtCo, Jul. 29, 2021, Helmenstine, Anne Marie, Ph.D. (2021, July 29). Definition of Radioactivity. Retrieved from Helmenstine, Anne Marie, Ph.D. "Definition of Radioactivity." ThoughtCo. (accessed June 4, 2023).