Geiger–Müller tube

Device for detecting alpha particles, beta particles, and gamma radiation, by measuring the electric current produced when these forms of radiation ionize a gas. The tube comprises an outer metal cylinder forming a negative terminal (cathode) with a thin membrane of plastic or mica covering the front of the tube. The inside of the tube is filled with argon gas and contains a metal rod at the centre forming a positive terminal (anode). Radiation from a sample travels through the thin membrane and ionizes the argon by removing electrons from the argon atoms. The electrons are attracted to the positively-charged rod. This flow of charge (an electric current) through the rod is converted to a digital reading on a Geiger counter.