10.2 - Ionisation Chambers

Ionisation chambers the 'gold standard' for absolute dosimetry.

Principles of ionisation chambers

When ionising radiation interacts with air, it produces ions (typically electrons and positively charged atoms from which they arrived). In the presence of charge, the positive particles will migrate towards a negative terminal and vice versa.
Ionisation chambers utilise this principle. All types contain a positive and negative terminal, separated by a gas (typically air). As ions reach their respective terminals, they produce a current which is detectable by an electrometer. The charge used within an ionisation chamber (100-400 V) is sufficient to pull ions towards the terminals, but not excessive enough to accelerate them and create further ionisations (see Geiger Muller Counters ).
Ionisation chambers need to have correction factors applied for different beam energies. They are highly sensitive (down to individual ionisations) which makes them useful in a variety of applications.

Types of ionisation chambers

Cylindrical ionisation chambers

Cylindrical chambers are the most commonly used type of ion chamber. The chamber consists of a central wire surrounded by air, all enveloped within a cylinder which is rounded at one end. The central wire functions as one terminal, whereas the outer shell functions as the other. The chamber is connected to an electrical supply (to supply charge) and is open to air.

Parallel plate ionisation chambers

Parallel plate chambers are arranged vertically, with two electrode plates separated by an air gap. They are more sensitive when the two plates are perpendicular to the beam direction.

Well type ionisation chambers

A well chamber consists of a hollow cavity surrounded by three layers of electrodes. The outer and inner layer are continuous and have the same charge, whereas the inner layer contains the opposite charge (similar to the wire used in the cylindrical chamber).

Use of ionisation chambers

Cylindrical chambers are used for absolute dosimetry due to their accuracy - they are able to measure single ionisation events. They can also waterproofed allowing use in a water phantom.
Parallel plate chambers are frequently used with slab phantoms and are good for measuring dose at a small point - as long as they are lying perpendicular to the beam direction.
Well type chambers are used for brachytherapy sources. The brachytherapy source is placed within the well chamber, which then captures all the outgoing radiation.