This radiation is in the form of beta (β) particles, which are high-energy electrons or positrons ejected from a nucleus in a process known as beta decay. There are two forms of beta decay, β− and β+, which respectively give rise to the electron and positron.
In β+-decay—observed in proton-rich nuclei—a proton is converted into a neutron, a positron and an electron-type neutrino:
Due to the presence of the neutrino, the atom and the beta particle do not usually recoil in opposite directions. This observation is in fact what led Wolfgang Pauli to postulate the existence of neutrinos in order to prevent violation of conservation of energy and momentum laws. Beta decay is mediated by the weak nuclear force.
Beta particles generally have a range about ten times as far as alpha particles and an ionising power about a tenth of that of alpha particles. They are stopped completely by a few millimeters of aluminium.