SPWLA Glossary

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A property of unstable [[isotopes]]. which undergo spontaneous atomic readjustment with the liberation of particles and/or energy (e.g., [[alpha particle|alpha]] or [[beta particle|beta particles]], [[neutron|neutrons]], and [[gamma ray|gamma rays]]). Alpha and beta enmission change the chemical nature of the element involved. The loss of energy will result in the [[decay]] or transformation of the unstable isotope into a stable isotope; or [[transmutation]] into an isotope of another element, sometimes giving rise to emission of neutrons.
The process of radioactive decay is one of conversion of mass to energy in accordance with Einstein's relationship, E = mc<sup>2</sup>. Nearly all of the energy of emitted particles and [[photon|photons]] is converted to heat in the near vicinity of the radioactive parent. This is one means by which the temperature of the earth is maintained.