Ionization counters

X-rays may be detected either photographically or with an ionization counter. They have great penetrating power which increases with their frequency, and owing to this are used to photograph the interior of many solid objects, notably the human body and in monitoring for faults in construction. 

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. 

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. 

As stated in the introduction to this chapter, semiconductor detectors operate like ionization counters. In ionization counters (see Chap. 5), the charges... 

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.)...

There are two types of equipment used to detect radiation, ionization counters... 

Ionization counters detect the production of ions in matter. 

Two types of devices—ionization counters and scintillation counters—are used to count particles emitted from radioactive nuclei and other nuclear processes. Ionization counters detect the production of ions in matter. Scintillation counters detect the production of scintillations, or flashes of light. 

Geger counter a kind of ionization counter used to count particles emitted by radioactive nuclei. It consists of a metal tube filled with gas, such as argon. (21.3)... 

Fig. exp. 7.3 Variation of the charge induced at the anode with the applied voltage in an ionization counter... 

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