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 +, and down quarks, with a − 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 ( +  −  = 1). Neutrons, with no charge, have one up quark and two down quarks ( −  −  = 0). Via the weak interaction, quarks can change flavor from down to up, resulting in electron emission. Positron emission happens when an up quark changes into a down quark, effectively converting a proton to a neutron.
Nuclei which decay by positron emission may also decay by electron capture. For low-energy decays, electron capture is energetically favored by 2mc = , 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 fraction of positron emission. However, if the energy difference is less than 2mc, then positron emission cannot occur and electron capture is the sole decay mode. Certain otherwise electron-capturing isotopes (for instance, ) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.