Ionization chamber

The ionization chamber is the simplest of all gas-filled radiation detectors, and is widely used for the detection and measurement of certain types of ionizing radiation; X-rays, gamma rays and beta particles. Conventionally, the term "ionization chamber" is used exclusively to describe those detectors which collect all the charges created by direct ionisation within the gas through the application of an electric field.^[1] It only uses the discrete charges created by each interaction between the incident radiation and the gas, and does not involve the gas multiplication mechanisms used by other radiation instruments, such as the Geiger-Muller counter or the proportional counter.

Ion chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. They are widely used in the nuclear power industry, research labs, radiography and other medical radiation fields, and in environmental monitoring.

Principle of operation

An ionization chamber measures the charge from the number of ion pairs created within a gas caused by incident radiation.^[2] 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. This continual generation of charge produces an ionisation current, which is a measure of the total ionising dose entering the chamber. However, the chamber cannot discriminate between radiation types (beta or gamma) and cannot produce an energy spectrum of radiation.

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.^[1]

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.

Chamber types and construction

The following chamber types are commonly used.

Free-air chamber

This is a chamber freely open to atmosphere, where the fill gas is ambient air. The domestic smoke detector is a good example of this, where a natural flow of air through the chamber is necessary so that smoke particles can be detected by the change in ion current. Other examples are applications where the ions are created outside the chamber but are carried in by a forced flow of air or gas.

Vented chamber

These chambers are normally cylindrical and operate at atmospheric pressure, but to prevent ingress of moisture a filter containing a desiccant is installed in the vent line.^[3] 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".^[1] 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 ^[3] and correction factors can be applied for very accurate measurement applications.

Sealed low pressure chamber

These are similar in construction to the vented chamber but are sealed and operate at or around atmospheric pressure. They contain a special fill gas to improve detection efficiency as free electrons are easily captured in air-filled vented chambers by neutral oxygen which is electronegative, to form negative ions. These chambers also have the advantage of not requiring a vent and desiccant. The beta end window limits the differential pressure from atmospheric pressure that can be tolerated, and common materials are stainless steel or titanium with a typical thickness of 25 micron.^[4]

High pressure chamber

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.^[3]

Research and calibration chambers

Early versions of the Ion chamber were used by Marie and Pierre Curie in their original work in isolating radioactive materials. Since then the ion chamber has been a widely used tool in the laboratory for research and calibration purposes. To do this a wide variety of bespoke chamber shapes, some using liquids as the ionised medium, have been evolved and used.^[5] Ion chambers are used by national laboratories to calibrate primary standards, and also to transfer these standards to other calibration facilities.

Instrument types

Hand held

Ion chambers are widely used in hand held radiation survey meters to measure beta and gamma radiation. They are particularly preferred for high dose rate measurements and for gamma radiation they give good accuracy for energies above 50-100 keV.^[1]

There are two basic configurations; the "integral" unit with the chamber and electronics in the same case, and the "two-piece" instrument which has a separate ion chamber probe attached to the electronics module by a flexible cable.

The chamber of the integral instrument is generally at the front of the case facing downwards, and for beta/gamma instruments there is a window in the bottom of the casing. This usually has a sliding shield which enables discrimination between gamma and beta radiation. The operator closes the shield to exclude beta, and can thereby calculate the rate of each radiation type.

Some hand held instruments generate audible clicks similar to that produced by a G-M counter to assist operators, who use the audio feedback in radiation survey and contamination checks. As the ion chamber works in current mode, not pulse mode, this is synthesised from the radiation rate.

Installed

For industrial process measurements and interlocks with sustained high radiation levels, the ion chamber is the preferred detector. In these applications only the chamber is situated in the measurement area, and the electronics are remotely situated to protect them from radiation and connected by a cable. Installed instruments can be used for measuring ambient gamma for personnel protection and normally sound an alarm above a preset rate, though the Geiger Muller tube instrument is generally preferred where high levels of accuracy are not required.

General precautions in use

Moisture is the main problem that affects the accuracy of ion chambers. The chamber's internal volume must be kept completely dry, and the vented type uses a desiccant to help with this.^[3] Because of the very low currents generated, any stray leakage current must be kept to a minimum in order to preserve accuracy. Invisible hygroscopic moisture on the surface of cable dielectrics and connectors can be sufficient to cause a leakage current which will swamp any radiation-induced ion current. This requires scrupulous cleaning of the chamber, its terminations and cables, and subsequent drying in an oven. "Guard rings" are generally used as a design feature on higher voltage tubes to reduce leakage through or along the surface of tube connection insulators which can require a resistance in the order of 10¹³Ohms.

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 10¹¹ count events.^[1] Additionally, the Geiger Muller tube cannot operate above about 10⁴ counts per second, due to dead time effects, whereas there is no similar limitation on the ion chamber.

Smoke detectors

The ionization chamber has found wide and beneficial use in smoke detectors. In a smoke detector, ambient air is allowed to freely enter the ionisation chamber. The chamber contains a small amount of americium-241, which is an emitter of alpha particles which produce a constant ion current. If smoke enters the detector, it disrupts this current because ions strike smoke particles and are neutralized. This drop in current triggers the alarm. The detector also has a reference chamber which is sealed but is ionised in the same way. Comparison of the ion currents in the two chambers allows compensation for changes due to air pressure, temperature, or the ageing of the source.

Medical radiation measurement

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. A chamber will have a calibration factor established by a national standards laboratory such as 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.

See also

• Absorbed dose
• Dosimetry
• Sievert chamber
• Stopping power of radiation particles

References