Positron emission

Positron emission is a type of beta decay, sometimes referred to as "beta plus" (β⁺). In beta plus decay, a proton is converted, via the weak force, to a neutron, a positron (also known as the "beta plus particle", the antimatter counterpart of an electron), and a neutrino.

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:

:

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 by less than m_"e", do not spontaneously decay by positron emission.

Nuclei which decay by positron emission may also decay by electron capture. For low-energy decays, electron capture is energetically favored by 2m_"e"c² = 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 2m_"e"c², then positron emission cannot occur and electron capture is the sole decay mode. Certain isotopes (for instance, SimpleNuclide|link|Beryllium|7) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.