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beta particle, one of the three types of radiation resulting from natural radioactivity. Beta radiation (or beta rays) was identified and named by E. Rutherford, who found that it consists of high-speed electrons. Unlike alpha and gamma particles, whose energy can be explained as the difference of the energies of the radioactive nucleus before and after emission, beta particles emerge with a variable energy. This apparent violation of the law of conservation of energy (see conservation laws) led to the hypothesis that a second undetected particle, the neutrino, is emitted along with the electron and shares the total available energy. In some forms of induced, or artificial, radioactivity, the electron's antiparticle, the positron, is emitted from the excited nucleus; the positron in this case is also called a beta particle and denoted by  + (the ordinary beta particle is -).
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