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.

Additional recommended knowledge

Don't let static charges disrupt your weighing accuracy

What is the Sensitivity of my Balance?

Safe Weighing Range Ensures Accurate Results

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 β⁺ and a neutrino ν_e:

These isotopes are used in positron emission tomography, a technique used for medical imaging.

Nuclei which decay by positron emission may also decay by electron capture. For low-energy decays, electron capture is energetically favored by 2m_ec² = 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_ec², 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.