The Geiger-Miller detector is a radiation detector which operates in the G-M region.

EO 2.6

DESCRIBE the operation of a Geiger-Miiller (G-M) detector to include:

a. Radiation detection b. Quenching

c. Positive ion sheath

The Geiger-Miiller or G-M detector is a radiation detector that operates in Region V, or G-M region, as shown on Figure 23. G-M detectors produce larger pulses than other types of detectors. However, discrimination is not possible, since the pulse height is independent of the type of radiation. Counting systems that use G-M detectors are not as complex as those using ion chambers or proportional counters.

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Figure 23 Gas Ionization Curve

The number of electrons collected by a gas-filled detector varies as applied voltage is increased. Once the voltage is increased beyond the proportional region, another flat portion of the curve is reached; this is known as the Geiger-Muller region. The Geiger-Muller region has two important characteristics:

The number of electrons produced is independent of applied voltage.

The number of electrons produced is independent of the number of electrons produced by the initial radiation.

This means that the radiation producing one electron will have the same size pulse as radiation producing hundreds or thousands of electrons. The reason for this characteristic is related to the way in which electrons are collected.

When a gamma produces an electron, the electron moves rapidly toward the positively charged central wire. As the electron nears the wire, its velocity increases. At some point its velocity is great enough to cause additional ionizations. As the electrons approach the central wire, the additional ionizations produce a larger number of electrons in the vicinity of the central wire.

As discussed before, for each electron produced there is a positive ion produced. As the applied voltage is increased, the number of positive ions near the central wire increases, and a positively charged cloud (called a positive ion sheath) forms around the central wire. The positive ion sheath reduces the field strength of the central wire and prevents further electrons from reaching the wire. It might appear that a positive ion sheath would increase the effect of the positive central wire, but this is not true; the positive potential is applied to the very thin central wire that makes the strength of the electric field very high. The positive ion sheath makes the central wire appear much thicker and reduces the field strength. This phenomenon is called the detector's space charge. The positive ions will migrate toward the negative chamber picking up electrons. As in a proportional counter, this transfer of electrons can release energy, causing ionization and the liberation of an electron. In order to prevent this secondary pulse, a quenching gas is used, usually an organic compound.

The G-M counter produces many more electrons than does a proportional counter; therefore, it is a much more sensitive device. It is often used in the detection of low-level gamma rays and beta particles for this reason. Electrons produced in a G-M tube are collected very rapidly, usually within a fraction of a microsecond. The output of the G-M detector is a pulse charge and is often large enough to drive a meter without additional amplification. Because the same size pulse is produced regardless of the amount of initial ionization, the G-M counter cannot distinguish radiation of different energies or types. This is the reason G-M counters are not adaptable for use as neutron detectors. The G-M detector is mainly used for portable instrumentation due to its sensitivity, simple counting circuit, and ability to detect low-level radiation.

Summary

The operation of Geiger-Miller detectors are summarized below.

G-M Detector Summary

The voltage of a Geiger-Miller (G-M) detector is set so that any incident radiation produces the same number of electrons.

As long as voltage remains in the G-M region, electron production is independent of operating voltage and the initial number of electrons produced by the incident radiation.

The operation voltage causes a large number of ionizations to occur near the central electrode as the electrons approach.

The large number of positive ions form a positive ion sheath which prevents additional electrons from reaching the electrode.

A quenching gas is used in order to prevent a secondary pulse due to ionization by the positive ions.