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Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an atomic number that is reduced by two.
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. [5] They are generally produced in the process of alpha decay but may also be produced in different ways.
On Earth, most naturally occurring helium-4 is produced by the alpha decay of heavy elements in the Earth's crust, after the planet cooled and solidified. When liquid helium-4 is cooled to below 2.17 K (−270.98 °C), it becomes a superfluid , with properties very different from those of an ordinary liquid.
Over Earth's history alpha-particle decay of uranium, thorium and other radioactive isotopes has generated significant amounts of 4 He, such that only around 7% of the helium now in the mantle is primordial helium, [17] lowering the total 3 He/ 4 He ratio to around 20 ppm.
Of these decay processes, only alpha decay (fission of a helium-4 nucleus) changes the atomic mass number (A) of the nucleus, and always decreases it by four. Because of this, almost any decay will result in a nucleus whose atomic mass number has the same residue mod 4. This divides the list of nuclides into four classes.
The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis. [113]
Any decay daughters that are the result of an alpha decay will also result in helium atoms being created. Some radionuclides may have several different paths of decay. For example, 35.94(6) % [ 27 ] of bismuth-212 decays, through alpha-emission, to thallium-208 while 64.06(6) % [ 27 ] of bismuth-212 decays, through beta-emission, to polonium-212 .
The most common isotope, 4 He, is produced on Earth by alpha decay of heavier elements; the alpha particles that emerge are fully ionized 4 He nuclei. 4 He is an unusually stable nucleus because it is doubly magic. It was formed in enormous quantities in Big Bang nucleosynthesis. Terrestrial helium consists almost exclusively (all but ~2ppm ...