|Nucleus · Nucleons (p, n) · Nuclear force · Nuclear reaction|
Positron emission or beta plus decay (β+ decay) is a particular type of radioactive decay and a subtype of beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (νe). Positron emission is mediated by the weak force. The positron is a type of beta particle (β+), the other beta particle being the electron (β-)emitted from the β- decay of a nucleus.
12Mg → 23
11Na + e+ + ν
Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large "proton-rich" radionuclides. Positron decay results in nuclear transmutation, changing an atom of a chemical element into an atom of an element with an atomic number that is less by one unit.
Positron emission should not be confused with electron 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. Electron capture (sometimes called inverse beta decay) is also occasionally classified as a type of beta decay. (In some ways, electron capture can be regarded as the "opposite" of positron emission. It can only occur when electrons are available and requires less energy difference between parent and daughter, so occurs much more often in smaller atoms than positron emission does.)
Discovery of positron emission 
In 1934 Frédéric and Irène Joliot-Curie bombarded aluminium with alpha particles to effect the nuclear reaction 4
2He + 27
13Al → 30
15P + 1
0n, and observed that the product isotope 30
15P emits a positron identical to those found in cosmic rays by Carl David Anderson in 1932. This was the first example of β+ decay (positron emission). The Curies termed the phenomenon "artificial radioactivity," since 30
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.
Positron-emitting isotopes 
Isotopes which undergo this decay and thereby emit positrons include carbon-11, potassium-40, nitrogen-13, oxygen-15, fluorine-18, and iodine-121. As an example, the following equation describes the beta plus decay of carbon-11 to boron-11, emitting a positron and a neutrino:
Emission mechanism 
Inside protons and neutrons, there are fundamental particles called quarks. The two most common types of quarks are up quarks, which have a charge of +2/3 and down quarks, with a −1/3 charge. Quarks arrange themselves in sets of three such that they make protons and neutrons. In a proton, whose charge is +1, there are two up quarks and one down quark. Neutrons, with no charge, have one up quark and two down quarks. Quarks are able to change from up quarks to down quarks. It is this that causes beta radiation. Positron emission happens when an up quark changes into a down quark.
Nuclei which decay by positron emission may also decay by electron 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 the branching ratio towards positron emission. However, if the energy difference is less than 2mec2, then positron emission cannot occur and electron capture is the sole decay mode. Certain isotopes (for instance, 7Be) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.
These isotopes are used in positron emission tomography, a technique used for medical imaging. Note that the energy emitted depends on the isotope that is decaying; the figure of 0.96 MeV applies only to the decay of carbon-11. 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.
- The University of North Carolina at Chapel Hill. "Nuclear Chemistry". Retrieved 2012-06-14.
- How it works:Positron emission
- Positron Emission Tomography Imaging at the University of British Columbia (accessed 11 May 2012)
- K. W. D. Ledingham et al., High power laser production of short-lived isotopes for positron emission tomography, Journal of Physics D: Applied Physics Volume 37 Number 16, 2004, 2341 doi:10.1088/0022-3727/37/16/019 (accessed 11 May 2012)
- The LIVEChart of Nuclides - IAEA with filter on β+ decay