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.
Hans Geiger developed the Geiger counter in 1928. The current version of the Geiger counter is called the halogen counter. It was invented in 1947 by Sidney H. Liebson. It has superseded the Geiger counter because of its much longer life and lower operating voltage.
The Geiger-Müller 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.