The most commonly used survey meter is the Geiger-Muller tube (Geiger Counter)
Principles of Geiger-Muller tube
The Geiger-Muller tube functions in a similar way to an ionisation chamber, but has a much higher voltage applied between its positive and negative electrode. Instead of air, the GM tube contains a noble gas (usually argon). The voltage is so great that ionisations occuring in the gas cause further ionisations as they are accelerate towards the electrode. These new ions go on to liberate further ions, and so on, causing an avalanche of ionisations. This rush of ions reach the electrode and generate a signal.
Once the avalanche has started, there is a dead time when ionisations cause no effect. This takes 50 - 300 microseconds to resolve and is due to the large build up of ions neutralising the electric field.
Use of the Geiger-Muller tube
The large signal from a single ionsiation means that a GM tube is able to detect small amounts of radiation and give a signal.
The dead time means that the ionisations can not be quantified.
Therefore, the GM tube is a good survey tool because it can pick up small amounts of radiation but a poor dosimeter because it can not quantify the results. A GM tube is often used in concert with another type of dosimeter - the GM tube detects areas of increased radiation and the dosimeter quantifies the result.