Geiger tubes

A gas ionization detector consists of a tube filled with a high pressure gas and two electrodes. A tube filled with 2 MPa (20 atm) of xenon is common. The gas in the tube ionizes when x-rays pass through the tube causing a current to flow between a high voltage potential placed across the electrodes. This concept is similar to that used in a Geiger tube detector. Gas ionization detectors are utilized in some CT scanners. 

Fig. 2-2. Schematic diagram of Geiger-counter tube. A typical end-window Geiger tube. The nature of the window will depend on the kind of rays to be detected. This tube will operate under the conditions of Fig. 2-3.

Analytical lines used, BaL l (2.567 A), CaKa (3.359 A), ZnKcx (1.436 A). Molybdenum target, helium path, rock salt analyzing crystal, and Geiger tube were used. The x-ray tube was operated at 50 kv, 30 ma (Zn) and 40 ma (Ba, Ca). 

Obviously, the nucleation is a randon process which is amplified by subsequent deposition of many thousands of silver atoms before the surface is completely covered (if integrated over the time interval of monolayer formation the current in each pulse corresponds to an identical charge). Such an amplification of random processes is the only way they can be observed. This situation is quite analogous, for example, to radioactive decay where a single disintegration is followed, in a Geiger tube, by the flow of millions of electrons. ... 

Use the balance to measure out 10.0 g salt substitute or pure potassium chloride (KC1). Pour the substance into the center of the petri dish so that it forms a small mound. Place the Student Radiation Monitor on top of the petri dish so that the Geiger Tube is positioned over the mound. Repeat step 5 until you have at least five data points. 

Geiger counter. Also known as a scintillation counter. A device used to detect, measure, and record radiation. The instrument gets its name from one of its parts, the Geiger tube, which is a gas-fiUed tube containing coaxial cylindrical electrodes. 

Neon is also used in scintillation counters, neutron fission counters, proportional counters, and ionization chambers for detection of charged particles. Its mixtures with bromine vapors or chlorine are used in Geiger tubes for counting nuclear particles. Helium-neon mixture is used in gas lasers. Some other applications of neon are in antifog devices, electrical current detectors, and lightning arrestors. The gas is also used in welding and preparative reactions. In preparative reactions it provides an inert atmosphere to shield the reaction from air contact. 

When the two Geiger tubes are arranged so that one is directly above the other and they are about one foot apart, the counter registers a number of particles, about ten per minute, which seem to go in a nearly vertical direction through both tubes. If the tubes are put horizontally side by side about one foot apart, very few particles are counted. This shows clearly that most of the particles are moving in a nearly vertical direction. 

A radioactive method has also been developed for determination of catalyst levels in reactors, hoppers, and catalyst-feed lines of moving-bed units (331). The equipment consists of one or more radiation sources (radium chloride), a Geiger tube, and associated electronic instruments. The radium sources are mounted within the vessel at intervals over the range of levels to be measured, with the Geiger tube at the top. The exact amounts and locations of the radium sources are so proportioned that radiation from each source is no longer detected when covered by a 2-ft. layer of catalyst. Thus the counter receives maximum radiation when the vessel is empty and decreasing intensities as catalyst level rises. 

Because most, but not all, emitted X-rays come from the surface of the wafer, a wafer thickness measurement is made prior to each XRF analysis to enable a wafer thickness correction factor to be applied. The equipment for measuring wafer thickness is made up of an Fe-55 nuclear source and a Geiger tube radiation detector (Figure 5). Thickness measurements of the known standards and shipboard sediment wafers are carried out by positioning the wafer between the nuclear source and the radiation detector and recording the attenuation of the X-ray radiation as it passes through the sediment sample. A comparison of the wafer data to that of the calibrated samples enables rapid, nondestructive determination of the thickness of the sediment samples (Figure 6). 

Slices of gel can be dried on adhesive labels and transferred to planchets for counting in a gas-flow counter. Loening (1968) describes a method for processing a large number of samples. Slices are dried on 16 mm cine film, placing one slice on every other frame. The film is fed under a Geiger tube using a cine camera or projector mechanism and the counts for each slice are thus printed out. 

FIGURE 19.4 In a Geiger tube, radiation ionizes gas in the tube, freeing electrons that are accelerated to the anode wire in a cascade. Their arrival creates an electrical pulse, which is detected by a ratemeter. The ratemeter displays the accumulated pulses as the number of ionization events per minute. 

Geiger tubes are usually filled with argon a low concentration of an organic quem hini gas. often alcohol or methane, is also present to minimize the production of secondary electrons when the cations strike the chamber wall. The lifetime of a lube is limited to some 10 to 10 counts, when the quencher becomes depleted. 

Eio. 9. Schematic ropre.sentation of the two-crystal small angle apparatus showing the defining slit, fixed crystal, specimen position, analyzing crystal, Geiger tube, and optical path. 

Sometimes it is possible in mixed decay to observe preferentially the decay of one species by proper choice of detection technique. For example, a proportional coimter may be used at an operating voltage that allows detection of ct-decay > 0) but excludes detection of /3-decay = 0). By contrast a typical Geiger counter can be used for /3-decay but does not detect a-r ation since the a-particles do not penetrate the window of the Geiger tube. These problems are discussed more extensively in Ch. 8. 

A gas flow Geiger tube was used to count the and radiation from the films. The radiation from and H can be distinguished because the energies of the beta-particles emitted in each case, and thus their penetrating powers, are different. C radiation was detected through a thin Mylar window on the Geiger tube. H radiation does not penetrate this window. The window was removed to count the H and C radiation combined. The net counting rate for radiation was ob-... 

In most techniques for studying adsorption on metals, xmiform, clean, and reproducible metal surfaces are difficult to prepare and the adsorption process cannot be followed continuously [2, 3,4,7,10,11,16, 18,2l]. Clean and reproducible metal surfaces are also difficult to prepare and maintain in methods that measure adsorption continuously and directly on a metal-coated window of a Geiger tube [l, 6,7,13]. A recently developed apparatus and technique provide controlled conditions for the production and maintenance of relatively clean metal films and the precise measurement of adsorption [20j. Metal is evaporated onto a mica window supported within a high-vacuum apparatus adsorption onto the metal film is measured directly and continuously by a counter tube below the window. 

Mica-window Geiger tubes were used to count the 1 cm foils and the glass tubes were used with the large indium foils. The small foils were placed on a flat aluminum plate beneath the mica window tubes, the large foils were enclosed in a brass cylinder and slipped over the glass tubes. Only the side of the foil which had faced the reactor was counted, and this side was always placed nearest the counting tube. 

There are many active detectors of ionizing radiation including Geiger tubes. 

From the above it is clear that each quenching event results in the destruction of the quencher. The Geiger tube therefore should become useless after some time. This is true. Most commercial tubes become useless after 10 to 10 ° pulses. 

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