Posita

Meet Posita, a proton who turns into a neutron and throws out a positron and a neutrino.

In some nuclei with too many protons, a proton changes into a neutron. Charge must balance, so a positron, the positively charged antimatter twin of the electron, is emitted, together with an electron neutrino. This is beta-plus decay. Irène Joliot-Curie and Frédéric Joliot made the first artificial positron emitters in 1934, earning the 1935 Nobel Prize in Chemistry.

A free proton cannot do this, because a neutron is heavier. Inside a nucleus the energy comes from the nucleus itself. For neutral atoms, the parent must outweigh the daughter by at least 2 × 0.511 = 1.022 MeV of rest energy, because of the extra electron mass involved.

PET scanners rely on this. A tracer containing fluorine-18, half-life about 110 minutes, emits positrons; each meets an electron and the pair annihilate into two gamma rays travelling in opposite directions, which the scanner detects.

The atomic number falls by one while the mass number stays the same. When the energy is too small for beta-plus, the nucleus can instead capture one of its own electrons.