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A Geiger counter (, GY-gər; also known as a Geiger–Müller counter or G-M counter) is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental physics and the nuclear industry. It detects ionizing radiation such as alpha particles, beta particles, and gamma rays using the ionization effect produced in a Geiger–Müller tube, which gives its name to the instrument. In wide and prominent use as a hand-held radiation survey instrument, it is perhaps one of the world's best-known radiation detection instruments. The original detection principle was realized in 1908 at the University of Manchester, but it was not until the development of the Geiger–Müller tube in 1928 that the Geiger counter could be produced as a practical instrument. Since then, it has been very popular due to its robust sensing element and relatively low cost. However, there are limitations in measuring high radiation rates and the energy of incident radiation. Principle of operation A Geiger counter consists of a Geiger–Müller tube (the sensing element which detects the radiation) and the processing electronics, which display the result. The Geiger–Müller tube is filled with an inert gas such as helium, neon, or argon at low pressure, to which a high voltage is applied. The tube briefly conducts electrical charge when high energy particles or gamma radiation make the gas conductive by ionization. The ionization is considerably amplified within the tube by the Townsend discharge effect to produce an easily measured detection pulse, which is fed to the processing and display electronics. This large pulse from the tube makes the Geiger counter relatively cheap to manufacture, as the subsequent electronics are greatly simplified. The electronics also generate the high voltage, typically 400–900 volts, that has to be applied to the Geiger–Müller tube to enable its operation. This voltage must be carefully selected, as too high a voltage will allow for continuous discharge, damaging the instrument and invalidating the results. Conversely, too low a voltage will result in an electric field that is too weak to generate a current pulse. The correct voltage is usually specified by the manufacturer. To help quickly terminate each discharge in the tube a small amount of halogen gas or organic material known as a quenching mixture is added to the fill gas. Readout There are two types of detected radiation readout: counts and radiation dose. The counts display is the simplest, and shows the number of ionizing events detected, displayed either as a count rate, such as "counts per minute" or "counts per second", or as a total number of counts over a set time period (an integrated total). The counts readout is normally used when alpha or beta particles are being detected. More complex to achieve is a display of radiation dose rate, displayed in units such as the sievert, which is normally used for measuring gamma or X-ray dose rates. A Geiger–Müller tube can detect the presence of radiation, but not its energy, which influences the radiation's ionizing effect. Consequently, instruments measuring dose rate require the use of an energy compensated Geiger–Müller tube, so that the dose displayed relates to the counts detected. The electronics will apply known factors to make this conversion, which is specific to each instrument and is determined by design and calibration. The readout can be analog or digital, and modern instruments offer serial communications with a host computer or network. There is usually an option to produce audible clicks representing the number of ionization events detected. This is the distinctive sound associated with handheld or portable Geiger counters. The purpose of this is to allow the user to concentrate on manipulation of the instrument while retaining auditory feedback on the radiation rate. Limitations There are two main limitations of the Geiger counter: Because the output pulse from a Geiger–Müller tube is always of the same magnitude (regardless of the energy of the incident radiation), the tube cannot differentiate between radiation types. The tube is less accurate at high radiation rates, because each ionization event is followed by a "dead time", an insensitive period during which any further incident radiation does not result in a count. Typically, the dead time will reduce indicated count rates above about 104 to 105 counts per second, depending on the characteristic of the tube being used. While some counters have circuitry which can compensate for this, for accurate measurements ion chamber instruments are preferred for high radiation rates. Types and applications The intended detection application of a Geiger counter dictates the tube design used. Consequently, there are a great many designs, but they can be generally categorized as "end-window", windowless "thin-walled", "thick-walled", and sometimes hybrids of these types. Particle detection The first historical uses of the Geiger principle were to detect α- and β-particles, and the instrument is still used for this purpose today. For α-particles and low energy β-particles, the "end-window" type of a Geiger–Müller tube has to be used, as these particles have a limited range and are easily stopped by a solid material. Therefore, the tube requires a window which is thin enough to allow as many as possible of these particles through to the fill gas. The window is usually made of mica with a density of about 1.5–2.0 mg/cm2. α-particles have the shortest range, and to detect these the window should ideally be within 10 mm of the radiation source due to α-particle attenuation. However, the Geiger–Müller tube produces a pulse output which is the same magnitude for all detected radiation, so a Geiger counter with an end window tube cannot distinguish between α- and β-particles. A skilled operator can use varying distance from a radiation source to differentiate between α- and high energy β-particles. The "pancake" Geiger–Müller tube is a variant of the end-window probe, but designed with a larger detection area to make checking quicker. However, the pressure of the atmosphere against the low pressure of the fill gas limits the window size due to the limited strength of the window membrane. Some β-particles can also be detected by a thin-walled "windowless" Geiger–Müller tube, which has no end-window, but allows high energy β-particles to pass through the tube walls. Although the tube walls have a greater stopping power than a thin end-window, they still allow these more energetic particles to reach the fill gas. End-window Geiger counters are still used as a general purpose, portable, radioactive contamination measurement and detection instrument, owing to their relatively low cost, robustness and relatively high detection efficiency; particularly with high energy β-particles. However, for discrimination .... Discover the A V Geiger popular books. Find the top 100 most popular A V Geiger books.