Geiger counters

As a result of its clever design, a Geiger counter offers an excellent option for obtaining concrete values for the radiation load. In principle, it uses the ionizing properties of nuclear radiation, from which it generates a measurement of the radiation load.

A Geiger counter detects nuclear radiation by means of a metal counter tube, which is at the same time a cathode, and inside the tube is a wire that functions as the anode. The counter tube is filled with argon or xenon, noble gases that cannot form anions. When ionizing radiation strikes a noble gas atom, an electron is knocked out of its orbit and immediately moves toward the anode. In the process, it is able to knock additional electrons out from the noble gas atoms (impact ionization). The noble gas atom cations ultimately move toward the cathode, where they take up electrons. As a result of this impact ionization, which essentially serves as amplification, a brief, measurable current arises in the Geiger counter that is proportional to the strength of the radiation.

The larger the voltage difference between the cathode and the anode, the greater the energy in the electron, and the number of impact ionizations increases. The voltage, which thus also affects the sensitivity of the device, must be controlled judiciously. Since in principle it is analogous to a commercial neon light tube, permanent ionization can take place, which would destroy the counter tube.

An additional problem, however, is the measurement of α- and β-radiation; this requires a window in the counter tube that allows the nuclear particles to enter but does not allow the gas to escape. Mica or a PET film are most frequently used for this purpose.

Older units can in some cases develop a leak, and the counter tube will no longer contain any noble gas. A good way to check the functioning is to measure the usual natural background radiation. An exact calibration can only be performed by measuring a standard radiation source. This service is offered by commercial service providers, and should be scheduled once per year.

In the better quality devices, the counter tube can be separated from the electronics, and replaced if necessary.

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