A Geiger–Müller tube (or GM tube) is the sensing element of a Geiger counter instrument that can detect a single particle of ionizing radiation, and typically produce an audible click for each. It was named for Hans Geiger who invented the device in 1908, and Walther Müller who collaborated with Geiger in developing it further in 1928. It is a type of gaseous ionization detector. The Geiger counter is sometimes used as a hardware random number generator
Description and operation
A Geiger–Müller tube consists of a tube filled with a low-pressure (~0.1 atm) inert gas such as helium, neon or argon (usually neon), in some cases in a Penning mixture, and an organic vapor or a halogen gas. The tube contains electrodes, between which there is a potential difference of several hundred volts, but no current flowing. The walls of the tube are either entirely metal or have their inside surface coated with a conductor to form the cathode while the anode is a wire passing up the center of the tube. When ionizing radiation passes through the tube, some of the gas molecules are ionized, creating positively charged ions, and electrons. The strong electric field created by the tube's electrodes accelerates the ions towards the cathode and the electrons towards the anode. The ion pairs gain sufficient energy to ionize further gas molecules through collisions on the way, creating an avalanche of charged particles. This results in a short, intense pulse of current which passes (or cascades) from the negative electrode to the positive electrode and is measured or counted. Most detectors include an audio amplifier that produce an audible click on discharge. The number of pulses per second measures the intensity of the radiation field. Some Geiger counters display an exposure rate (e.g. mR/h), but this does not relate easily to a dose rate as the instrument does not discriminate between radiation of different energies.
The Geiger plateau is the voltage range in which the Geiger–Müller counter operates. If a GM tube is exposed to a steady radiation source and the applied voltage increased from zero, at first the count rate increases rapidly; at a certain voltage the rate of increase flattens out (only changing a few per cent for every 100 volts increase). Depending on the characteristics of the specific tube (manufacturer, size, gas type etc.) the exact voltage range may vary. In this plateau region, the potential difference in the counter is strong enough to ionize all the gas inside the tube, upon triggering by the incoming ionizing radiation (alpha, beta or gamma radiation). Below the plateau the voltage is not high enough to cause complete discharge; a limited Townsend avalanche is the result, and the tube acts as a proportional counter, where the output pulse size depends on the initial ionization created by the radiation. Higher voltages give a pulse size independent of the initial ionization energy. If the applied voltage is too high, a continuous glow discharge is formed and the tube cannot detect radiation. The plateau has a slight incline caused by increased sensitivity to low energy radiation, due to the increased voltage on the device. Normally when a particle enters the tube and ionizes one of the gas atoms, complete ionization of the gas occurs. Once a low energy particle enters the counter, it is possible that the kinetic energy in addition to the potential energy of the voltage are insufficient for the additional ionization to occur and thus the ion recombines. At higher voltages, the threshold for the minimum radiation level drops, thus the counter's sensitivity rises. The counting rate for a given radiation source varies slightly as the applied voltage is varied; for standardization of the response of the instrument, a regulated voltage is used to maintain stable counting characteristics.
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