Positron emission
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| Nuclear processes |
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Radioactive decay processes
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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, 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 β+ and a neutrino νe:
These isotopes are used in positron emission tomography, a technique used for medical imaging.
Positron decay occurs when there are too many protons in the nucleus of an atom. There must also be an energy difference between initial and final states of at least twice the rest energy of an electron (2mec2, or 1.022 MeV). Half of this value, 0.511 MeV, is necessary to create the positron. The other half provides the missing mass necessary to convert the proton to a neutron. The remaining decay energy appears as kinetic energy distributed among the positron, neutrino, and recoil nucleus.
Electron capture is a competing decay mode to this process and is energetically favored, but as the energy difference 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 are effectively stable in cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission. For example, 7Be decays in the laboratory by electron capture, but it is a stable component of cosmic rays.
