Henri Becquerel demonstrated beta particles in 1900. Identical with electrons is there negative charge at -1. Their mass is 549 millionths of one AMU, 1/2000 of the mass of a proton or neutron. Beta particles consist of high energetic electrons emitted by radioactive nuclei or neutrons. By the process of beta decay, one of the neutrons in the nucleus is transformed into a proton and a new atom is formed which has one less neutron but one more proton in the core. Beta decay is accompanied by the emission of a positron (the antiparticle of the electron), a positive charged antineutrino. Beta particles have a greater range of penetration than alpha particles but less than gamma rays or x-rays. The name beta was coined by Rutherford in 1897. The traveling speed of beta particles depends on their energy. Because of their small mass and charge beta particles travel deep into tissues and cause cellular damage and possible cancer.

See also Radiation Shielding.

There are two kinds of beta decay: beta minus and beta plus decay. The differentiation depends on the charge of the emitted particle.
At the beta plus decay in the nucleus a proton changes to a neutron and emits a positron and a neutrino. The atom is after the decay a different element, but with the same number of particles in the nucleus.
At the beta minus decay in the nucleus a neutron changes to a proton and emits an electron and an antineutrino. As with the beta plus decay the atom changes to a different element but with the same number of particles in the nucleus.
Sometimes the electron capture is mentioned as a third kind of beta decay.
Beta decay is used for example in positron-electron tomography or in iodine-131 therapy.

See also Electron Capture.