Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
the chamber walls, where they receive an electron and are neutral once again.
The central wire is usually the anode, which is positive with respect to the chamber walls;
hence electrons are attracted to it. Electrons, however, are very light, and a small force
applied to them (by the voltage V in the detector) gives them a lot of energy. Therefore,
the voltage applied in the chamber must be low enough to keep the electrons from
acquiring so much energy before reaching the anode that the electrons themselves ionize
other atoms. This type of ionization is called secondary ionization. Usually ionization
chambers have voltages between 100 and 300 V, although some have higher operating
voltages.
2.4.1.2. Geiger Counters
Geiger counters, also called Geiger‐Muller or G‐M tubes, are widely used as monitoring
instruments because they can detect any type of radiation that produces ionization within
the detector, no matter how small the amount of ionization. In fact, this high sensitivity
makes Geiger counters ideally suited for detecting fast electrons and photons because the
low probability of these particles producing ionization makes them hard to detect. Alpha
particles and other highly ionizing particles can be detected by G‐M tubes, but because of
their short range they are unable to penetrate the walls of the detector unless thin
windows are used or the source of the radiation is placed inside the detector.
Geiger counters are almost always used in pulse mode. Their operation requires that the
number of electrons collected at the anode be independent of the amount of primary
ionization. Hence this factor cannot be used as a measure of particle energy, nor is it
possible to discriminate among different types of particles by means of the sensitivity of
the electronic circuit. Therefore, G‐M tubes can tell nothing about the energy of radiation.
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