|Fig. 1: Hans Geiger, 1928 (Source: Wikimedia Commons).|
The Geiger Counter, also known as the Geiger-Müller tube, is an instrument used for the detection and measurement of different types of radiation. Its use as a hand-held radiation survey device has made it one of the most famous radiation particle detectors in the world.  The relatively low cost, extreme sensitivity, and user-friendly design of the Geiger Counter have made it the preferred tool for physicists, medical professionals, and those in the nuclear industry. [1-3] However, its design is not perfect as it has limitations measuring high-energy photons that can be picked up by scintillation counters.
The original device was designed in 1908 by German physicist Hans Geiger (Fig.1) and British Physicist Ernest Rutherford at the University of Manchester, but it was only capable of detecting α particles. [2,3] It was only in 1928 that Geiger and one of his PhD students, Walther Müller, improved the counter by developing the Geiger-Müller Tube, allowing the instrument to detect other types of ionizing radiations.
The apparatus consists of the counter, the tube, and a power supply that is able to generate enough voltage for operation. [2-4] The metal tube contains a thin metal wire running along its middle, which is sealed off and filled with an inert gas such as helium, argon, or neon. [2-4] A high voltage is applied and maintained on the wire relative to the tube, creating an electric field on the inside. [3,4] When ionizing radiation (α, β, or γ particles) enters the tube and ionizes the atoms of the inert gas, an electron is lost, leaving the atom positively charged.  The knocked-off electrons gain energy, collide with other atoms, and release more electrons until the process snowballs into an easily detectable pulse of current, which is counted by the device. After this pulse is counted, the previously charged ions become neutralized and the counter is reset, ready to record again. [1-4] The counts can be displayed as a number on a readout or as an audible click, the distinctive sound associated with many Geiger Counters, that allows the user to continue to operate the mechanism while receiving auditory feedback on the radiation rate.
The versions of the Geiger Counter used today incorporate improvements that allow it a longer operating life and a lower required voltage.
Changes and and improvements have since been made to the Geiger Counter so that it may be of use in many professional fields.  The tube itself is modified for different purposes; a larger, windowless tube is used for X-ray and γ radiation detection due to its increased gas volume, although the efficiency of the device for this purpose is relatively low.  The gas within the tube can also be varied, which allows the operator to then detect and count neutrons. The handheld device is used widely to measure and detect radioactive contamination, the cheapness and effectiveness of the device in detecting β particles has allowed it to remain relevant in the physics, medical, and nuclear industries. [1-4]
However, scintillation and proportional counters are becoming increasingly popular due to their ability to differentiate between particle types and their higher degrees of sophistication and reliability. Even so, the Geiger counter has been an important invention and instrument in nuclear physics, geo-physics, and medical therapy.
© Peter Russo. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. T. Krebs, "Early History of the Scintillation Counter," Science 122, 3157 (1955).
 A. T. Krebs, "Hans Geiger, Fiftieth Anniversary of the Publication of His Doctoral Thesis, 23 July 1906," Science, 124, 3213 (1956).
 R. Lemmerman, "Operation of the Geiger Counter Tube and Its Associated Equipment," Am. Biol. Teach., 27, 6 (1965).
 S. C. Curran, "The New Counters," Sci. Prog., 42, 165 (1954).