When these electrons neutralize the ionized molecules of the quenching
gas, energy is released, which causes the dissociation of the molecules of the
gas but with no UV emissions to prolong the avalanche. This prevents the
continuous discharge of the GM counter. Organic molecules are more effec-
tive quenchers but dissociate irreversibly and therefore give a limited life-
time for the GM tube (~10^8 –10^10 pulses). In contrast, dissociated inorganic
molecules recombine to form the original molecules, and therefore halogen-
quenched GM tubes have infinite useful lifetime.
The probes of GM counters can be either end-window type or side-
window type. An end-window–type GM counter is shown in Figure 7.4. The
window is made of thin mica (0.01 mm thick), and gases such as argon,
methane, helium, and neon mixed with halogen are commonly used as the
counting gas. The typical gas pressure in GM probes is about 0.8 atmos-
phere. Different shapes of GM probes are available, such as cylindrical and
pancake types. Some GM probes are provided with a metal cover that stops
all b-particles and very low-energy g-radiations so that only high-energy
photons are detected. Without the cover, both b-particles and g-rays are
detected. The GM counter is usually battery operated at a voltage of
500–900 V. Lower voltages are used for smaller tubes, and some special
tubes are operated even at 1300 V. The meter connected to the GM probe
gives readings in mR/hr or counts per minute. Some counters are equipped
with audible alarms or flashing light alarms that are triggered by radiation
above a preset intensity. The latter counter is often used to monitor the radi-
ation level in work areas and is called an area monitor.
78 7. Gas-Filled Detectors
Fig. 7.4. An end-window–type Geiger–Müller counter. (Courtesy of Nuclear
Associates, Carle Place, NY.)