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. Geiger counters are used to detect ionizing radiation, usually beta particles and gamma rays, but certain models can detect alpha particles. An inert gas-filled tube (usually helium, neon or argon with halogens added) briefly conducts electricity when a particle or photon of radiation makes the gas conductive. The tube amplifies this conduction by a cascade effect and outputs a current pulse, which is then often displayed by a needle or lamp and/or audible clicks. Modern instruments can report radioactivity over several orders of magnitude. Some Geiger counters can be used to detect gamma radiation, though sensitivity can be lower for high energy gamma radiation than with certain other types of detectors. The density of gas in the device is usually low, allowing most high energy gamma photons to pass through undetected. Lower energy photons are easier to detect, and are better absorbed by the detector. Examples of this are the X-ray and Beta Pancake Geiger Tube. Good alpha and beta scintillation counters also exist, but Geiger detectors are still favored as general purpose alpha/beta/gamma portable contamination and dose rate instruments, due to their low cost and robustness. A variation of the Geiger tube is used to measure neutrons, where the gas used is boron trifluoride or Helium 3 and a plastic moderator is used to slow the neutrons. This creates an alpha particle inside the detector and thus neutrons can be counted.
Types and applications
A GM instrument is one of many different types of radiation detectors. The Geiger–Müller tube is one form of a class of radiation detectors called gaseous detectors or simply gas detectors. Although useful, cheap and robust, a counter using a GM tube can only detect the presence and intensity of radiation (number of particles detected in an interval of time, as opposed to energy or wavelength). The Geiger–Müller counter has applications in the fields of nuclear physics, geophysics (mining), and medical therapy with isotopes and x-rays. Some of the proportional counters have many electrodes and are called multi-wire proportional counters or simply MWPCs.
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