Geiger counter
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Geiger counter
orGeiger-Müller (G-M) counter
(gī`gər-mŭl`ər, –myo͞o`lər), instrument for the detection and quantitative determination of ionizing radiation such as the alpha and beta rays given off by radioactive minerals and cosmic rays. It was first developed by Hans (J. W.) GeigerGeiger, Johannes Wilhelm(Hans Geiger) , 1882–1945, German physicist. Geiger received a doctorate in physics at Erlangen in 1906, then went to Manchester, where he assisted British chemist Ernest Rutherford.
..... Click the link for more information. and later improved by Geiger and A. Müller. Variously designed for different uses, it consists commonly of a gas-filled metal cylinder that acts as one electrode, and a needle or thin taut wire along the axis of the cylinder that acts as the other electrode. Glass caps used to seal the ends of the tube serve as insulators. A voltage applied to the device is so adjusted that it is almost strong enough to cause a current to pass through the gas from one electrode to the other. The gas becomes ionized whenever the counter is brought near radioactive substances, however little the quantity and however faint the emanations. The resulting ionized particles of gas are able to carry the current from one electrode to the other, thus completing a circuit. Once established, the current is amplified by an electronic device so that it can indicate by an audible click the presence of ionized particles. The gas quickly returns to its normal nonionized state, permitting each new particle or ray to register, making counting possible. The instrument can also register ionization by a pointer and scale called a rate meter. The Geiger counter is used in the detection of cosmic rays and for locating radioactive minerals. Counters enable radioactive tracerstracer,
an identifiable substance used to follow the course of a physical, chemical, or biological process. In chemistry the ideal tracer has the same chemical properties as the molecule it replaces and undergoes the same reactions but can at all times be detectible and
..... Click the link for more information. to be followed as they make their way through complex organisms such as the human body; in medicine Geiger counters have found several successful uses in the location of malignancies. They are used also to follow radioactive isotopes in chemical reactions. For a number of research applications the Geiger counter has been largely replaced by scintillometers and other more complex devices.
Geiger Counter
(also Geiger-Müller counter), a gas-discharge instrument for the detection and study of various types of radioactive and other ionizing radiation: alpha and beta particles, gamma quanta, light and X-ray quanta, neutrons, and high-energy particles. Gamma quanta are detected on a Geiger counter by their secondary ionizing particles, such as photoelectrons, Compton electrons, and electron-positron pairs. Neutrons are detected by the recoil nuclei and by the nuclear reaction products generated in the gas of the counter.
The working volume of a Geiger counter is a gas-discharge space with a strongly nonuniform electrical field. The most frequently used counters have coaxially arranged cylindrical electrodes. The cathode is the outer cylinder, the anode is a thin wire stretched along its axis. The electrodes are hermetically enclosed in a reservoir filled with some gas under a pressure of up to 13-26 kilonewtons per sq m (kN/m2), or 100-200 millimeters of mercury (mm Hg). A potential difference of several hundred volts is applied to the counter electrodes. The wire is positively charged through the resistance R. If the working volume of the counter does not contain free electrons, an electrical discharge is not formed within the volume. An ionizing particle penetrating into the counter generates free electrons in the gas that move toward the positively charged wire. The electrical field intensity near the wire is high, and the electrons are accelerated to such an extent that they, in turn, begin to ionize the gas. Consequently there is an avalanche-like increase in the number of electrons with decreasing distance from the wire. A corona discharge flareup is generated, and a current flows through the counter. At a sufficiently high R (108-1010 ohm), a negative charge is accumulated on the wire, and the potential difference between the wire and the cathode decreases rapidly, with the result that the discharge is terminated. After this, the sensitivity of the counter is restored in 10-1—10-3 sec (the discharge time of the capacitor C through the resistance R). Such a long insensitivity time is inconvenient in many applications. In view of this, nonself-quenching counters, in which the resistance R quenches the charges, were replaced by self-quenching counters (proposed by Trost), which are also more stable. In these counters, the discharge is quenched spontaneously, even at low values of resistance R, because of a special gas filling (inert gas with an admixture of complex molecules—for example, alcohol vapor—and small admixtures of halogens, such as chlorine, bromine, or iodine). The insensitivity time of a self-quenching counter is ~10-4 sec.
The electrical signals in the external circuit that arise during the discharge flareups in a Geiger counter are amplified and recorded by an electromagnetic counter or a scaling system. As the voltage V applied to the counter increases, the number N of impulses recorded per unit time increases sharply until the voltage reaches the value VΔ and then levels off; it begins to rise sharply again at the value VB. The operating region of the characteristic curve (the plateau) is several dozen to several hundred volts long. On the plateau, the number of counts is virtually equal to the number of ionizing particles that enter the counter.
Geiger counters are used in many areas of physics, in biology and medicine, and in archaeology, geology, and engineering.
REFERENCES
Printsipy i Metody registratsii elementarnykh chastits. Moscow, 1963. (Translated from English.)Kalashnikova, V. I., and M. S. Kozodaev. Detektory elementarnykh chastits. Moscow, 1966. (Eksperimental’nye metody iadernoi fiziki, part 1.)