Gas ionization counters

Gas Ionization Counters A common gas ionization counter is the Geiger-Muller counter where the electronic pulses derived from the ionization process are registered as counts. The instrument can be adjusted to detect only radiation with a desired penetrating power. 

In INAA, a rock or mineral sample is irradiated in the reactor. The irradiated sample is removed from the reactor, and the dangerous radioactivities are allowed to decay. Then the sample is placed into a counter and the y-rays emitted by each element in the sample are counted. A variety of counters are used, including scintillation counters, gas ionization counters, or semi-conductor counters. For the most precise results, background counts in the detectors produced by electronic noise, cosmic rays, and other radioactive decays must be eliminated. The technique is very sensitive, and samples as small as a few tens of milligrams can be measured. 

A common gas ionization counter is the Geiger-Miiller counter (Figure 26-5). Radiation enters the tube through a thin window. Windows of different stopping powers can be used to admit only radiation of certain penetrating powers. 

Figure 26-5 The principle of operation of a gas ionization counter. The center wire is positively charged, and the shell of the tube is negatively charged. When radiation enters through the window, it ionizes one or more gas atoms. The electrons are attracted to the central wire, and the positive ions are drawn to the shell. This constitutes a pulse of electric current, which is amplified and displayed on the meter or other readout.

Geiger-MtiUer counter A type of gas ionization counter used to detect radiation. 

Excitation of sample by bombardment with electrons, radioactive particles or white X-rays. Dispersive crystal analysers dispersing radiation at angles dependent upon energy (wavelength), detection of radiation with gas ionization or scintillation counters. Non-dispersive semiconductor detectors used in conjunction with multichannel pulse height analysers. Electron beam excitation together with scanning electron microscopes. 

Gas ionization detectors are widely used in radiochemistry and X-ray spectrometry. They are simple and robust in construction and may be employed as static or flow detectors. Flow studies have received attention in the interfacing of radioactive detectors with gas chromatographs. A radio-gas chromatograph (Figure 10.9) uses a gas flow proportional counter to monitor the effluent from the gas chromatography column. To achieve... 

TOWNSEND AVALANCHE. A term used in gas-filled counter technology to describe a process which is essentially a cascade multiplication of ions. In this process an ion produces another ion by collision, and the new and original ions produce still others by further collisions, resulting finally in an avalanche of ions (or electrons). The terms cumulative ionization and cascade are also used to describe this process. It occurs in a nonself-maintained gas discharge, where ions have sufficient energy. 

Putting aside all of the above details of semiconductor physics, we can regard a Si(Li) counter simply as a solid-state ionization chamber, with one difference. X-rays incident on a gas ionization chamber produce a constant current (Sec. 7-5). In a Si(Li) counter the current flows in discrete pulses, because the voltage is high enough to sweep the counter free of charge carriers (the electrons and holes are highly mobile) before the next incident photon creates new carriers. 

Gas-filled counters (ionization, proportional, Geiger-Muller counters)... 

Normally, gas counters are manufactured to operate in one region only. The user buys an ionization counter, a proportional counter, or a GM counter. The manufacturer has selected the combination of variables 1-4 listed above that results in the desired type of gas counter. The last variable, the high voltage applied, is not a fixed number, but a range of values. The range is specified by the manufacturer, but the user decides on the best possible value of HV. 

Fano factors have been calculated and also measured. For semiconductor detectors, F values as low as 0.06 have been reported. For gas-filled counters, reported F values lie between 0.2 and 0.5. Values of f < 1 mean that the generation of electron-hole pairs does not exactly follow Poisson statistics. Since Poisson statistics applies to outcomes that are independent, it seems that the ionization events in a counter are interdependent. 

Energy distributions in gas proportional counters— whether gas flow or sealed counters— undergo a noticeable shift to lower energies when X-ray counting rates (and, concomitantly, detector gas ionization rates) increase. The exact origin of this phenome-... 

Figure 3. Plot of counter gas ionization efficiency vs. X-ray wavelength for Ar (dashed line) and Xe (solid line) detector gases. Positions of the three L absorption edges for Xe and the single K absorption edge for Ar are indicated. Figure adapted from Goldstein et al. (1984).

All gas-filled counters are in principle ion chambers (with the exception of the less common gas scintillation counters). The ionization produced in an ion chamber by a single nuclear particle produces too low a charge pulse to be easily detectable exc t for a-particles. However, an ion chamber can be designed so that the munber of ion pairs formed in each event is multiplied greatly. 

Figure 8.22 Gas-filled detector response vs. potential. A detector operating at the plateau marked B is an ionization counter. A proportional counter operates in the sloping region marked C where the response is proportional to the energy of the incoming photon. The plateau marked D represents the response of a Geiger counter. (Modified from Helsen and Kuczumow, used with permission.)...

When an X-ray falls on a semiconductor, it generates an electron (—e) and a hole (+e) in a fashion analogous to the formation of a primary ion pair in a proportional counter. Based on this phenomenon, semiconductor detectors have been developed and are now of prime importance in EDXRF and scanning electron microscopy. The principle is similar to that of the gas ionization detector as used in a proportional counter, except that the materials used are in the solid-state. The total ionization caused by an X-ray photon striking the detector is proportional to the energy of the incident photon. 

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