A high-speed electron or positron emitted in the decay of a radioactive isotope.
Electron ejected with great velocity from a radioactive atom that is undergoing spontaneous disintegration. Beta particles do not exist in the nucleus but are created on disintegration, beta decay, when a neutron converts to a proton to emit an electron.
Beta particles are more penetrating than alpha particles, but less so than gamma radiation; they can travel several meters in air, but are stopped by 2–3 mm of aluminum. They are less strongly ionizing than alpha particles and, like cathode rays, are easily deflected by magnetic and electric fields.