ionization chamber

An instrument for detecting ionizing radiation by measuring the amount of charge liberated by the interaction of ionizing radiation with suitable gases, liquids, or solids.

While the gold leaf electroscope is the oldest form of ionization chamber, instruments of this type are still widely used as monitors of radiations by workers in the nuclear or radiomedical professions. However, for many purposes it is useful to measure the ionization pulse produced by a single ionizing particle. See also Electroscope.

The simplest form of a pulse ionization chamber consists of two conducting electrodes in a container filled with gas (see illustration). A battery, or other power supply, maintains an electric field between the positive anode and the negative cathode. When ionizing radiation penetrates the gas in the chamber—entering, for example, through a thin gas-tight window—this radiation liberates electrons from the gas atoms leaving positively charged ions. The electric field present in the gas sweeps these electrons and ions out of the gas, the electrons going to the anode and the positive ions to the cathode.

Parallel-plate ionization chamber.

In a chamber, such as that represented in the illustration, the current begins to flow as soon as the electrons and ions begin to separate under the influence of the applied electric field. The time it takes for the full current pulse to be observed depends on the drift velocity of the electrons and ions in the gas. Because the ions are thousands of times more massive than the electrons, the electrons always travel several orders of magnitude faster than the ions. As a result, virtually all pulse ionization chambers make use of only the relatively fast electron signal.

One of the most important uses of an ionization chamber is to measure the total energy of a particle or, if the particle does not stop in the ionization chamber, the energy lost by the particle in the chamber. In addition to energy information, ionization chambers are now routinely built to give information about the position within the gas volume where the initial ionization event occurred. This information can be important not only in experiments in nuclear and high-energy physics where these position-sensitive detectors were first developed, but also in medical and industrial applications.

Foremost among the other applications is the use of gas ionization chambers for radiation monitoring. Portable instruments of this type usually employ a detector containing approximately 60 in.³ (1 liter) of gas, and operate by integrating the current produced by the ambient radiation. Another application of ionization chambers is the use of air-filled chambers as domestic fire alarms. Yet another development in ion chamber usage is that of two-dimensional imaging in x-ray medical applications to replace the use of photographic plates.

Gaseous ionization chambers have also found application as total-energy monitors for high-energy accelerators. Such applications involve the use of a very large number of interleaved thin parallel metal plates immersed in a gas inside a large container.

Ionization chambers can be made where the initial ionization occurs, not in gases, but in suitable liquids or solids. In the solid-state ionization chamber (or solid-state detector) the gas filling is replaced by a large single crystal of suitably chosen solid material. In this case the incident radiation creates electron-hole pairs in the crystal, and this constitutes the signal charge. Silicon and germanium detectors have proved to be highly successful and have led to detectors that have revolutionized low-energy nuclear spectroscopy. The use of a liquid in an ionization chamber combines many of the advantages of both solid and gas-filled ionization chambers; most importantly, such devices have the flexibility in design of gas chambers with the high density of solid chambers. During the 1970s a number of groups built liquid argon ionization chambers and demonstrated their feasibility.

An instrument for measuring the quantity of ionizing radiation, in terms of the charge of electricity associated with ions produced within a defined volume of air.

The ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation. Conventionally, the term "ionisation chamber" is used exclusively to describe those detectors which collect ion pairs from gases.^[1]

Contents 1 Construction and operation 2 Smoke detectors 3 Radiation measurement 4 References 5 See also

Construction and operation

An ionization chamber is an instrument constructed to measure the number of ions within a medium (which we will consider to be gaseous, but can also be solid or liquid). It consists of a gas filled enclosure between two conducting electrodes. The electrodes may be in the form of parallel plates (Parallel Plate Ionization Chambers: PPIC), or coaxial cylinders to form a convenient portable detector; in some cases one of the electrodes may be the wall of the vessel itself.

When gas between the electrodes is ionized by any means, such as by alpha particles, beta particles, X-rays, or other radioactive emission, the ions and dissociated electrons move to the electrodes of the opposite polarity, thus creating an ionization current which may be measured by a galvanometer or electrometer. Each ion essentially deposits or removes a small electric charge to or from an electrode, such that the accumulated charge is proportional to the number of like-charged ions. A voltage potential that can have a wide range from a few volts to many kilovolts can be applied between the electrodes; depending on the application. The applied voltage allows the device to work continuously by mopping up electrons and preventing the device from becoming saturated. The current that originates is called a bias current, and prevents the device from reaching a point where no more ions can be collected.

Ionization chambers are widely used in the nuclear industry as they provide an output that is proportional to radiation dose and have a greater operating lifetime than standard Geiger tubes; as in Geiger-Müller tubes the gas eventually breaks down.
Ionization chambers are used in nuclear medicine to determine the exact activity of radioactive therapeutic treatments. Such devices are called 'radioisotope dose calibrators'. Ion chambers are sometimes microphonic as they are very sensitive devices and non-ion related charges can be set up inside due to the piezoelectric effect.

Smoke detectors

The ionisation chamber has found wide and beneficial use in smoke detectors. In a smoke detector, the gap between the plates is exposed to the open air. The chamber contains a small amount of americium-241, which is an emitter of alpha particles. These alpha particles carry a substantial amount of energy, and when they collide with gas in the ionization chamber (mostly nitrogen and oxygen) the momentum transferred ionizes the gas molecules—that is, the uncharged gas molecules will lose one or more electrons and become charged ions.

Since the plates are at different voltages (in a typical smoke detector, the voltage difference is a few volts) the ions and electrons will be attracted to the plates. This small flow of ions between the plates represents a measurable electric 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.

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. Ionization chambers are connected to electrometers, and they typically report a collected charge in nanocoulombs. A correction factor is then required to convert this reading into a meaningful dose. Often, a chamber will have a factor established by a national standards laboratory such as the NPL in the UK, or will have a factor determined by comparison against a standards-calibrated chamber at the user's site.

References

1. ^ Glenn F Knoll, Radiation detection and measurement pub 1999. ISBN 0471 073385

See also

• Dosimetry

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