n (1904) A subatomic particle created at the instant of emission from a decaying radioactive atomic nucleus, having a mass (at rest) of 9.1095 × 10−28 g. A negatively charged beta particle is identical to an ordinary electron, and a positively charged one is identical to a positron. A stream of beta particles is called a beta ray. Such rays are use in equipment for measuring and controlling the thickness of plastics films, sheets, and other extrudates. (Serway RA, Faugh JS, Bennett CV (2005) College physics. Thomas, New York).