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High-energy processes |
Positron emission,beta plus decay, orβ+ decay is a subtype ofradioactive decay calledbeta decay, in which aproton inside aradionuclide nucleus is converted into aneutron while releasing apositron and anelectron neutrino (νe).[1] Positron emission is mediated by theweak force. The positron is a type ofbeta particle (β+), the other beta particle being the electron (β−) emitted from the β− decay of a nucleus.
An example of positron emission (β+ decay) is shown withmagnesium-23 decaying intosodium-23:
Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large "proton-rich" radionuclides. Positron decay results innuclear transmutation, changing an atom of one chemical element into an atom of an element with anatomic number that is less by one unit.
Positron emission occurs extremely rarely in nature on Earth. Known instances includecosmic ray interactions and the decay of certainisotopes, such aspotassium-40. This rare form of potassium makes up only 0.012% of the element on Earth and has a 1 in 100,000 chance of decaying via positron emission[2].
Positron emission should not be confused withelectron emission or beta minus decay (β− decay), which occurs when a neutron turns into a proton and the nucleus emits an electron and an antineutrino.
Positron emission is different fromproton decay, the hypothetical decay of protons, not necessarily those bound with neutrons, not necessarily through the emission of a positron, and not as part of nuclear physics, but rather ofparticle physics.
In 1934Frédéric andIrène Joliot-Curie bombarded aluminium withalpha particles (emitted bypolonium) to effect the nuclear reaction4
2He + 27
13Al →30
15P + 1
0n, and observed that the product isotope30
15P emits a positron identical to those found in cosmic rays byCarl David Anderson in 1932.[3] This was the first example ofβ+
decay (positron emission). The Curies termed the phenomenon "artificial radioactivity", because30
15P is a short-lived nuclide which does not exist in nature. The discovery of artificial radioactivity would be cited when the husband-and-wife team won the Nobel Prize.
Isotopes which undergo this decay and thereby emit positrons include, but are not limited to:carbon-11,nitrogen-13,oxygen-15,fluorine-18,copper-64, gallium-68, bromine-78,rubidium-82, yttrium-86, zirconium-89,[4]sodium-22,aluminium-26,potassium-40,strontium-83, andiodine-124.[4][5] As an example, the following equation describes the beta plus decay of carbon-11 toboron-11, emitting a positron and aneutrino:
Inside protons and neutrons, there arefundamental particles calledquarks. The two most common types of quarks areup quarks, which have a charge of +2⁄3, anddown quarks, with a −1⁄3 charge. Quarks arrange themselves in sets of three such that they makeprotons andneutrons. In a proton, whose charge is +1, there are twoup quarks and onedown quark (2⁄3 +2⁄3 −1⁄3 = 1). Neutrons, with no charge, have oneup quark and twodown quarks (2⁄3 −1⁄3 −1⁄3 = 0). Via theweak interaction, quarks can changeflavor fromdown toup, resulting inelectron emission. Positron emission happens when anup quark changes into adown quark, effectively converting a proton to a neutron.[6]
Nuclei which decay by positron emission may also decay byelectron capture. For low-energy decays, electron capture is energetically favored by 2mec2 =1.022 MeV, since the final state has an electron removed rather than a positron added. As the energy of the decay goes up, so does thebranching fraction of positron emission. However, if the energy difference is less than 2mec2, the positron emission cannot occur and electron capture is the sole decay mode. Certain otherwise electron-capturing isotopes (for instance,7
Be) are stable ingalactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.
A positron is ejected from the parent nucleus, but the daughter (Z−1) atom still has Z atomic electrons from the parent, i.e. the daughter is a negative ion (at least immediately after the positron emission). Since tables of masses are foratomic masses,, and, since the mass of the positron is identical to that of the electron, the overall result is that the mass-energy oftwo electrons is required, and the β+ decay is energetically possibleif and only if the mass of the parent atom exceeds the mass of the daughter atom by at least two electron masses (2me c2 = 1.022 MeV).[7]
Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2me, cannot spontaneously decay by positron emission.[7]
These isotopes are used inpositron emission tomography, a technique used for medical imaging. The energy emitted depends on the isotope that is decaying; the figure of0.96 MeV applies only to the decay ofcarbon-11.
The short-lived positron emitting isotopes11C (T1⁄2 =20.4 min),13N (T1⁄2 =9.9 min),15O (T1⁄2 =2.0 min), and18F (T1⁄2 =109.8 min) used for positron emission tomography are typically produced by proton or deuteron irradiation of natural or enriched targets.[8][9]
Live Chart of Nuclides: nuclear structure and decay data (main decay modes) - IAEA
