Beta particles are high-energy electrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the greek letter (β).
There are two forms of beta decay, β− and β+, which respectively give rise to the electron and the positron.
β− decay (electron)
Unstable atomic nuclei with an excess of neutrons may undergo β− decay, where a neutron is converted into a proton, an electron and an electron-type antineutrino (the antiparticle of the neutrino):
The electron and the antineutrino are emitted from the nucleus.
β+ decay (positron)
Unstable nuclei that are deficient in neutrons may undergo β+ decay, where a proton is converted into a neutron, a positron and an electron-type neutrino:
The positron and the neutrino are emitted from the nucleus.
The neutrino and conservation of energy
Due to the presence of the neutrino, the atom and the beta particle do not usually recoil in opposite directions. This observation 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 can be stopped by a few mil (1/1000 in.) of aluminum and its flight is ten times farther than an alpha particle although it ionizes a tenth less than an alpha particle.