n (beta gauge) A device for measuring the thickness of plastics films, sheets, or extruded shapes, consisting of a source of beta rays and a detecting element. When material is passed between the source and the detector, some of the rays are absorbed, the percent absorbed being a measure of the thickness of the material. Signals from the detecting element can be used to control equipment that automatically regulates the thickness. The most usual sources are krypton 85 and strontium 90. Also used for particular applications are cesium 137, promethium 147, and ruthenium 106. See also Thickness Gauging.