Principle of Operation
An ionization chamber measures the charge from the number of 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 arrangement 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 between the electrodes is ionized by incident ionising radiation, Ion-pairs are created and the resultant positive ions and dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field. This generates an ionization current which is measured by an electrometer circuit. The electrometer must be capable of measuring the very small output current which is in the region of femtoamperes to picoamperes, depending on the chamber design, radiation dose and applied voltage.
Each ion pair created deposits or removes a small electric charge to or from an electrode, such that the accumulated charge is proportional to the number of ion pairs created, and hence the radiation dose.
The electric field also enables the device to work continuously by mopping up electrons, which prevents the fill gas from becoming saturated, where no more ions could be collected, and by preventing the recombination of ion pairs, which would diminish the ion current. 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-Muller tube or the proportional counter.
Referring to the accompanying ion pair collection graph, it can be seen that in the "ion chamber" operating region the collection of ion pairs 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-Muller tube or the proportional counter whereby secondary electrons, and ultimately multiple avalanches, greatly amplify the original ion-current charge.
Read more about this topic: Ionization Chamber
Famous quotes containing the words principle of, principle and/or operation:
“The principle of all sovereignty resides essentially in the nation.”
—French National Assembly. Declaration of the Rights of Man (Sept. 1791)
“I ... observed the great beauty of American government to be, that the simple machines of representation, carried through all its parts, gives facility for a being moulded at will to fit with the knowledge of the age; that thus, although it should be imperfect in any or all of its parts, it bears within it a perfect principle the principle of improvement.
”
—Frances Wright (1795–1852)
“An absolute can only be given in an intuition, while all the rest has to do with analysis. We call intuition here the sympathy by which one is transported into the interior of an object in order to coincide with what there is unique and consequently inexpressible in it. Analysis, on the contrary, is the operation which reduces the object to elements already known.”
—Henri Bergson (1859–1941)