A Geiger counter, also called a Geiger–Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a Geiger Müller tube. Each particle detected produces a pulse of current, but the Geiger counter cannot distinguish the energy of the source particles. Invented in 1908, Geiger counters remain popular instruments used for measurements in health, physics, industry, geology and other fields, because they can be made with simple electronic circuits.
In Wasteland, it is used to detect when you are getting close to irradiated squares on the map, such as the southern route to Darwin Village or the radioactive moat in Base Cochise. Christina comes equipped with one, so it is not necessary to buy one, though the Quartz Emporium sells them for $300 if you decide not to (H)ire her.