ionization chamber

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ionization chamber,

device for the detection and measurement of ionizing radiationradiation
, term applied to the emission and transmission of energy through space or through a material medium and also to the radiated energy itself. In its widest sense the term includes electromagnetic, acoustic, and particle radiation, and all forms of ionizing radiation.
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. It consists basically of a sealed chamber containing a gas and two electrodes between which a voltage is maintained by an external circuit. When ionizing radiation, e.g., a photonphoton
, the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent.
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, enters the chamber (through a foil-covered window), it ionizes one or more gas molecules. The ions are attracted to the oppositely charged electrodes; their presence causes a momentary drop in the voltage, which is recorded by the external circuit. The observed voltage drop helps identify the radiation because it depends on the degree of ionization, which in turn depends on the charge, mass, and speed of the photon. See radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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.

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 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 chamberenlarge picture
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.

Ionization Chamber

 

a device used in the study and recording of nuclear particles and radiation; its operation is based on the capability of fast charged particles to induce ionization of a gas. An ionization chamber is an air or gas electric capacitor to whose electrodes a potential difference V is applied. When ionizing particles enter the space between electrodes, electrons and ions of the gas are formed, which, as they move in the electric field, collect at the electrodes and are recorded by the recording equipment. An ionization chamber with parallel flat electrodes (disks) is the simplest type. The diameter of the disk is several times greater than the distance between the electrodes. In a cylindrical ionization chamber the electrodes are two coaxial cylinders, one of which is grounded and serves as the housing of the ionization chamber (Figure 1). A spherical ionization chamber consists of two concentric spheres (sometimes the inner electrode is a rod).

Figure 1. Cross section of a cylindrical ionization chamber: (1) cylindrical housing of the chamber, which serves as the negative electrode; (2) cylindrical rod, which is the positive electrode; (3) insulators

A distinction is made between current and counting ionization chambers. In current ionization chambers the intensity of the current / created by the electrons and ions is measured by a galvanometer (Figure 2). The dependence of / on V (Figure 3)—the current-voltage characteristic—has a horizontal section AB, on which the current does not depend on the voltage (the saturation current Io). This corresponds to total collection on the electrodes of the ionization chamber of all electrons and ions formed. The section AB usually is the operating region of the ionization chamber. Current ionization chambers provide data on the total number of ions formed per second. They usually are used to measure radiation intensity and to make dosimetric measurements. Since the ionization currents in an ionization chamber are usually small (10-10—10-15 amperes), they are intensified by DC amplifiers.

Figure 2. Connection diagram of a current ionization chamber:(V) voltage at electrodes of chamber, (G) galvanometer, which measures ionization current

Figure 3. Current-voltage characteristic of an ionization chamber

In counting ionization chambers the voltage pulses that arise at the resistor R (Figure 4) when the ionization current induced by the passage of each particle moves through it are recorded and measured. The amplitude and length of the pulses depend on the value of R and on the capacitance C (Figure 4). For a counting ionization chamber operating in the region of the saturation current, the pulse amplitude is proportional to the energy € lost by the particle within the ionization chamber. Counting ionization chambers are usually used to study strongly ionizing short-range particles that can come to a full stop in the space between the electrodes (a-particles and fragments of fissioning nuclei). In this case the magnitude of the ionization chamber pulse is proportional to the total energy of the particle, and the amplitude distribution of the pulses duplicates the energy distribution of the particles—that is, it gives the energy spectrum of the particles. The resolution, or the accuracy of the measurement of the energy of an individual particle, is an important characteristic of a counting ionization chamber. For an a-particle with an energy of 5 mega electron volts the resolution may reach 0.5 percent.

Figure 4. Connection diagram of a counting ionization chamber: (C) capacitance of passive electrode, (R) resistance

In the pulse mode the response time τ of the ionization chamber should be reduced as much as possible. By selecting the quantity Rit is possible to ensure that the pulses of the ionization chamber correspond to the collection only of electrons, which are much more mobile than ions. Here a great reduction of the pulse length and the achievement of τ ≃ 1 microsec are possible.

By varying the shape of the ionization chamber electrodes and the composition and pressure of the filling gas, optimal conditions for recording a specific type of radiation can be provided. To study short-range particles in an ionization chamber, the source is placed within the chamber, or small inlet ports made of mica or synthetic materials are cut in the housing. In an ionization chamber for the study of gamma radiation, ionization is accomplished by secondary electrons driven from atoms of the gas or the walls of the ionization chamber. The greater the volume of the ionization chamber, the more ions formed by the secondary electrons. Therefore to measure gamma radiation of low intensity, ionization chambers of large volume (several liters or more) are used.

An ionization chamber also can be used to measure neutrons. In this case ionization is accomplished by the recoil nuclei (usually protons) generated by fast neutrons as well as by a-particles, protons, or γ-quanta that form upon capture of slow neutrons byl0B,3He, or113Cd nuclei. These substances are introduced into the gas or the material of the walls of the ionization chamber. Gas-amplified ionization chambers are used for the study of particles that create a low ionization density. Ionization chambers also are used in the study of cosmic rays.

REFERENCES

Kalashnikova, V. I., and M. S. Kozodaev. Detektory elementarnykh chastits. Moscow, 1966.(Eksperimental’nye metody iadernoi fiziki, part 1.)
Al’fa-, beta- i gamma-spektroskopiia, issue 1. Edited by K. Siegbahn. Moscow, 1969. (Translated from English.)

K. P. MITROFANOV

ionization chamber

[‚ī·ə·nə′zā·shən ‚chām·bər]
(nucleonics)
A particle detector which measures the ionization produced in the gas filling the chamber by the fast-moving charged particles as they pass through. Also known as ion chamber.