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Radioactive decay is the process of spontaneous disintegration of some atomic nuclei, accompanied by the emission of radiation — typically photons, electrons or helium-4 nuclei — and the release of heat.
The average speed of radioactive decay is determined by the composition of the atomic nucleus — how many protons and neutrons it contains, and therefore to which nuclide it belongs. It is impossible to predict exactly when a particular atom will decay, but it is possible to predict how many atoms from a group will remain after a given time. The time in which on average half of the atoms in a sample disintegrate is called the half life and is specific to each radioactive nuclide. Radioactive decay is independent of pressure, temperature and chemical composition. This means that the rate of this process cannot be controlled in any way, which has both desirable and undesirable consequences.
The decay of radioactive nuclides is used for dating objects with varying levels of accuracy. This is called radiometric dating. Radioactive decay also releases heat, which can be quite substantial for some nuclides. This phenomenon is used to power deep space probes with the decay heat of plutonium-238 using devices called radioisotope thermoelectric generators.
Difference from nuclear fission
Radioactive decay is distinct from the process of nuclear fission, which is the process of disintegration of atomic nuclei initiated by irradiation with neutrons, accompanied by the release of radiation, heat and more neutrons. Unlike radioactive decay, the rate of nuclear fission can be controlled by varying the neutron flux. Nuclear fission is used in nuclear power and nuclear weapons.
Different radionuclei will decay in different ways. The modes by which a radioactive substance can decay include:
- Alpha emission (α). In alpha decay, a nucleus emits an alpha particle (a helium-4 nucleus), thereby giving up 2 of its protons and 2 of its neutrons. For example, radium-226 decays via α decay into radon-222.
- Beta- emission (β-). In beta-minus decay, a nucleus emits an electron. In the process, one of the neutrons in the nucleus transforms into a proton. This is the more common of the two beta decay modes, and as such is sometimes just called "beta decay" with the minus being understood. For example, cobalt-60 decays via β decay into nickel-60.
- Beta+ emission (β+). In beta-plus decay, a nucleus emits a positron (the antimatter counterpart to the electron). In the process, one of the protons in the nucleus transforms into a neutron. For example, carbon-11 decays via β+ decay into boron-11.
- Electron capture (EC, or K-capture). In electron capture, one of the electrons orbiting the nucleus is absorbed into the nucleus, turning one of the protons into a neutron. For example, aluminum-26 can decay via electron capture into magnesium-26. In general, most nuclei that can undergo β+ decay can also undergo electron capture. This is the only decay mode in which the nucleus's chemical environment, such as pressure, can have a measurable impact on its decay rate.
- Neutron emission (n). In neutron emission, one of the neutrons in the nucleus is released. The nucleus thus becomes a slightly lighter isotope of the same element. For example, helium-5 (whose half-life is less than 1 billionth of a picosecond) spontaneously decays into helium-4 by ejecting a neutron.
In all of these decay modes, neutrinos and high-energy photons (gamma rays) can also be emitted.
- With the exception of nuclides decaying by electron capture — their half lives can be varied by a few percent by modifying their chemical environment.