Editing Ionization chamber (section)

==Principle of operation==
[[File:Ion chamber operation.gif|thumb|right|300px|Schematic diagram of a parallel plate ion chamber, showing creation of ion pairs, and drift of ions due to electric field. Electrons typically drift 1000 times faster than positive ions due to their smaller mass.<ref name=knoll/>]]
[[File:Detector regions.gif|thumb|300px|Plot of ion current against voltage for a conceptual wire cylinder gaseous radiation detector. Ion chambers use the lowest voltage plateau.]]
A gas ionization chamber measures the charge from the number of [[ion|ion pairs]] created within a gas caused by incident radiation. It consists of a gas-filled chamber with two [[electrodes]]; known as [[anode]] and [[cathode]]. The electrodes may be in the form of parallel plates (Parallel Plate Ionization Chambers: PPIC), or a cylinder with a coaxially located internal anode wire.

A [[voltage]] potential is applied between the electrodes to create an electric field in the fill gas. When gas atoms or molecules between the electrodes are ionized by incident [[ionizing radiation]], [[ionization|ion-pairs]] are created and the resultant positive ions and dissociated electrons move to the electrodes of the opposite [[chemical polarity|polarity]] under the influence of the electric field. This generates an ionization current which is measured by an [[electrometer]] circuit in the region of [[femtoampere]]s to [[picoampere]]s, depending on the chamber design, and is proportional to the radiation dose.<ref name=knoll/>

The electric field has to be sufficiently strong to prevent the [[plasma recombination|recombination]] of ion pairs which would diminish the ion current, and build-up of positive ions is prevented by their recombination with electrons when they reach the cathode. This mode of operation is referred to as "current" mode, meaning that the output signal is a continuous current, and not a pulse output as in the cases of the [[Geiger–Müller tube]] or the proportional counter.<ref name=knoll/> 

Referring to the accompanying ion-pair collection graph, it can be seen that in the ion chamber operating region the charge of a collected ion pair is effectively constant over a range of applied voltage, as due to its relatively low electric field strength the ion chamber does not have any multiplication effect. This is in distinction to the Geiger–Müller tube or the proportional counter whereby secondary electrons, and ultimately multiple avalanches, greatly amplify the original ionisation charges to produce measurable pulses.<ref name=knoll/>