Beta particle

They generally have a range about ten times as far as Alpha particles and an ionising[?] power about a tenth as alpha's. They are stopped completely by a few millimeters of aluminium

They are high-energy electrons ejected from a nucleus, in a process known as beta decay, where a neutron decays into a proton, an electron and an anti-electron neutrino.

n⁰ => p⁺ + e^- + (ν_e-bar)

Due to the presence of the neutrino, the atom and the electron 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.

The electron gun inside a television tube could also be considered a source of beta radiation, which is stopped by the phosphors[?] inside the tube to create light.

see also Alpha particle, Gamma rays, Radioactivity, radiation physics,Nuclear physics