Beta Particles
Beta particles are energetic electrons that are emitted from the nucleus. They are born when a neutron decays to a proton. Since neutrons are neutral particles and protons are positive, conservation of charge requires a negatively charged electron to be emitted. Some isotopes decay by converting a proton to a neutron, thus emitting a positron (an anti-electron). These particles can penetrate matter more than can alpha particles, and it takes a small aluminum plate to stop most beta particles.