The Geiger counter is a device used to detect radiation. In the Modern Warfare series of games, whenever the player goes into a radioactive area, a clicking noise from the Counter is heard.
The Geiger counter is used in a few multiplayer maps in Call of Duty: Modern Warfare 2, such as Afghan and Wasteland, near the border of the map, where there is lethal radiation to prevent the player going out of the map. This mechanic is also used in the Special Ops mission, Hidden.
In Call of Duty: Warzone, the Detection contract replaces the player's tactical equipment with a Geiger counter. It is used to locate a Radioactive Cache, guaranteed to drop an RC-XD.