Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
The quality of a G‐M tube is determined by the counting rate versus voltage response, as
shown in Fig. (2.9). That range of operating voltages through which the counting rate
changes very little is called the plateau. A good tube has a counting‐rate change of less
than 10% throughout the plateau region. The voltage at which the G‐M tube begins to
count is known as the starting voltage. Voltages higher than the plateau voltage result in a
breakdown of the tube gas; i.e., ionization occurs because the voltage itself pulls the gas
atoms apart even without the presence of any ionizing radiation.
Fig. (2.9) Geiger Counter Plateau: A represents the starting potential or starting voltage, B
represents the “knee” or threshold voltage, C represents the plateau region, and D
represents the breakdown or continuous discharge region.
2.4.1.3. Proportional Counters
In the previous section we discussed the production of secondary and continued
ionization near the anode. In that previous case, virtually all the gas atoms in the sensitive
volume of the detector were ionized each time even one ion pair was created within it. All
dependence on the energy of the ionizing particle was lost. Referring back to Fig. (2.8),
we see there is a voltage region between the ionization chamber region and the G‐M tube
region designated as the proportional region. A very useful detector, known as a
proportional counter can be designed to operate in this region. Multiplication of
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