Any of three processes of radioactive disintegration in which a beta particle is spontaneously emitted by an unstable atomic nucleus in order to dissipate excess energy. Beta particles are either electrons or positrons. The three beta-decay processes are electron emission, positron emission, and electron capture. The process of beta decay increases or decreases the positive charge of the original nucleus by one unit without changing the mass number. Though beta decay is in general a slower process than gamma or alpha decay, beta particles can penetrate hundreds of times farther than alpha particles. Beta decay half-lives are a few milliseconds or more. See also radioactivity.
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