Beta radiation has a penetrating power greater than alpha radiation but less than gamma radiation. It can be used to control the thickness of materials that are made in sheets. Examples include paper and aluminium foil.
Paper moves between the rollers. If the paper gets too thick, the amount of radiation reaching the Giger Muller tube is reduced. The reduction is detected and electronic circuitry adjusts the roller spacing to that the paper gets thinner. Conversely, if the paper gets too thin, the amount of radiation reaching the Giger Muller tube increases. The increase is detected and electronic circuitry adjusts the roller spacing to that the paper gets thicker.
A radioactive material with a long half life is used so that it need not be replaced often.