A Geiger counter measures ionizing radiation. Geiger counters can detect alpha, beta, and gamma radiation, but not neutrons. The sensor is a Geiger-Müller tube, a gas-filled tube that briefly conducts electricity when a particle or photon of radiation briefly makes the gas conductive. The instrument amplifies this signal and displays it to the user.
The Geiger-muller tube is one form of a class of radiation detectors called ion chambers. Ion chambers instrumented to both detect radiation and determine particle energy levels are called Proportional counters.
Other devices detecting radiation include dosimeters, semiconductor diode detectors, scintillation counters, track detectors, cloud chambers, bubble chambers, spark chambers, neutron detectors and microcalorimeters.