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 β+ (the ordinary beta particle is β-).
An electron emitted by the nucleus of a radioactive atom. The beta particle is produced when a neutron within the nucleus decays into a proton and an electron. Beta particles have greater penetrating power than alpha particles but less than x-ray or gamma rays. Although beta particles can penetrate skin, they travel only a short distance in tissue. Beta rays pose relatively little health hazard, therefore, unless they are ingested into the body. Naturally radioactive materials such as potassium-40, carbon-14, and strontium-90 emit beta particles, as do a number of synthetic radioactive materials.
See also Radioactivity
be·ta par·ti·cle (also beta ray) • n. Physics a fast-moving electron emitted in radioactive decay.