Geiger Counter (or Geiger-Müller Counter) is a type of particle detector that measures ionizing radiation. Geiger Counter instruments 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. Geiger counters are popular instruments used for measurements in health physics, industry, geology and other fields, because they can be made with simple electronic circuits.

Digital Geiger Counter - Radioactivity Detector for Radiation Measurement & AnalysisA Geiger counter lets you check the environment and items for radioactivity. You can use to check for the presence of radon on your house or basement, or even use it to go prospecting for uranium or other radioactive minerals. Radioactivity is the emission of energy from the nucleus of certain nuclides or elements. Some naturally occuring radioactive elements include uranium and thorium and radon. A small amount of naturally occuring potassium is even radioactive. There are three types of radioactive emissions: Alpha, Beta rays, and Gamma rays.

A “Geiger counter” usually contains a metal tube with a thin metal wire along its middle, the space in between them sealed off and filled with a suitable gas, and with the wire at about +1000 volts relative to the tube. When there is an ion or electron penetrating the tube (or an electron knocked out of the wall by X-rays or gamma rays) tears electrons off atoms in the gas (usually helium, neon or argon with halogens added), and because of the high positive voltage of the central wire, those electrons are then attracted to it. In doing so they gain energy, collide with atoms and release more electrons, until the process snowballs into an “avalanche” which produces an easily detectable pulse of current. With a suitable filling gas, the flow of electricity stops by itself, or else the electrical circuitry can help stop it. 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 Geiger Counter 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 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. The Geiger-Müller counter has applications in the fields of nuclear physics, geophysics (mining), and medical therapy with isotopes and x-rays.