Gas-filled detectors include the ionisation chamber, the proportional counter and the Geiger counter;
They operate on the basis of ionisation of gas atoms by the incident radiation, where the positive ions and electrons produced are collected by electrodes;
An ion pair is the term used to describe a positive ion and an electron;
The operation of gas-filled detectors is critically dependent on the magnitude of the applied dc voltage;
The output voltage of an ionisation chamber can be calculated on the basis of the capacitance of the chamber;
A very sensitive amplifier is required to measure voltage pulses produced by an ionisation chamber;
The gas in ionisation chambers is usually air;
Ionisation chambers are typically used to measure radiation exposure (in a device called an Exposure Meter) and radioactivity (in a device called an Isotope Calibrator);
The total charge collected in a proportional counter may be up to 1000 times the charge produced initially by the radiation;
The initial ionisation triggers a complete gas breakdown in a Geiger counter;
The gas in a Geiger counter is usually an inert gas;
The gas breakdown must be stopped in order to prepare the Geiger counter for a new event by a process called quenching;
Two types of quenching are possible: electronic quenching and the use of a quenching gas;
Geiger counters suffer from dead time, a small period of time following the gas breakdown when the counter is inoperative;
The true count rate can be determined from the actual count rate and the dead time using an equation;
The value of the applied dc voltage in a Geiger counter is critical, but high stability is not required.
