Ionization chamber

The ionization chamber is the simplest type of gaseous ionization detector, and is widely used for the detection and measurement of many types of ionizing radiation, including X-rays, gamma rays, alpha particles and beta particles. Conventionally, the term "ionization chamber" refers exclusively to those detectors which collect all the charges created by direct ionization within the gas through the application of an electric field. It uses the discrete charges created by each interaction between the incident radiation and the gas to produce an output in the form of a small direct current. This means individual ionising events cannot be measured, so the energy of different types of radiation cannot be differentiated, but it gives a very good measurement of overall ionising effect.

It has a good uniform response to radiation over a wide range of energies and is the preferred means of measuring high levels of gamma radiation, such as in a radiation hot cell as they can tolerate prolonged periods in high radiation fields without degradation.

They are widely used in the nuclear power industry, research labs, fire detection, radiation protection, and environmental monitoring.

Principle of operation

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. This is to stop moisture building up in the interior of the chamber, which would otherwise be introduced by the "pump" effect of changing atmospheric air pressure. These chambers have a cylindrical body made of aluminium or plastic a few millimetres thick. The material is selected to have an atomic number similar to that of air so that the wall is said to be "air equivalent" over a range of radiation beam energies. This has the effect of ensuring the gas in the chamber is acting as though it were a portion of an infinitely large gas volume, and increases the accuracy by reducing interactions of gamma with the wall material. The higher the atomic number of the wall material, the greater the chance of interaction. The wall thickness is a trade-off between maintaining the air effect with a thicker wall, and increasing sensitivity by using a thinner wall. These chambers often have an end window made of material thin enough, such as mylar, so that [[beta particles can enter the gas volume. Gamma radiation enters both through the end window and the side walls. For hand-held instruments the wall thickness is made as uniform as possible to reduce photon directionality though any beta window response is obviously highly directional. Vented chambers are susceptible to small changes in efficiency with air pressure

High-pressure chamber

thumb|Two high pressure cylindrical ion chambers in an enclosure.

The efficiency of the chamber can be further increased by the use of a high-pressure gas. Typically a pressure of 8-10 atmospheres can be used, and various noble gases are employed. The higher pressure results in a greater gas density and thereby a greater chance of collision with the fill gas and ion-pair creation by incident radiation. Because of the increased wall thickness required to withstand this high pressure, only gamma radiation can be detected. These detectors are used in survey meters and for environmental monitoring.

For industrial applications with remote electronics, the ion chamber is housed in a separate enclosure which provides mechanical protection and contains a desiccant to remove moisture which could affect the termination resistance.

In installations where the chamber is a long distance from the measuring electronics, readings can be affected by external electromagnetic radiation acting on the cable. To overcome this a local converter module is often used to translate the very low ion chamber currents to a pulse train or data signal related to the incident radiation. These are immune to electromagnetic effects.

Applications

Nuclear industry

Ionization chambers are widely used in the nuclear industry as they provide an output that is proportional to radiation dose. They find wide use in situations where a constant high dose rate is being measured as they have a greater operating lifetime than standard Geiger–Müller tubes, which suffer from gas break down and are generally limited to a life of about 10<sup>11</sup> count events.

Medical radiation measurement

thumb|354x354px|Diagram of a nuclear medicine dose calibrator or radionuclide calibrator that uses a "well-type" ionization chamber. The dipper is used to give a reproducible source position. The radioactive substance in this example is liquid.

In medical physics and radiotherapy, ionization chambers are used to ensure that the dose delivered from a therapy unit or radiopharmaceutical is what is intended. The devices used for radiotherapy are called "reference dosimeters", while those used for radiopharmaceuticals are called radioisotope dose calibrators - an inexact name for radionuclide radioactivity calibrators, which are used for measurement of radioactivity but not absorbed dose. A chamber will have a calibration factor established by a national standards laboratory such as ARPANSA in Australia or the NPL in the UK, or will have a factor determined by comparison against a transfer standard chamber traceable to national standards at the user's site.

Guidance on application use

In the United Kingdom the HSE has issued a user guide on selecting the correct radiation measurement instrument for the particular application concerned. This covers all radiation instrument technologies, and is a useful comparative guide to the use of ion chamber instruments.