What Ions are Produced in an Ionization Chamber?

The ionization chamber is a gas-filled radiation detector and is widely used for the detection and measurement of nuclear particles and certain types of ionizing radiation; X-rays, γ rays and β particles. It is based on the principle of excitation or ionization of the atoms of the medium through which the incident charged particles pass. Charged particles, as they pass through matter, leave along their paths a chain of ionized or excited atoms that can be detected and counted. Most detectors measure ionization caused by the passage of a charged particle through a suitable material.

A simple ionization chamber consists of a metal cylinder with a thin axial wire enclosed in a glass envelope in which some inert gas is filled. A high potential difference is established between the cylinder and the electrode (wire). When a charged particle enters the active volume (i.e. gas) of the chamber, it produces a large number of ion pairs in the enclosed gas along its path.

Suppose that n pairs of ions are produced in the chamber, then ne electrons will be attracted to the anode and positive ne+ ions will be attracted to the cathode. The current signal or voltage pulse, therefore, developed through R is proportional to the number of electrons collected by the electrode. The ionizing event is thus recorded by the amplifier. However, the amplifiers used can only record pulses of small magnitude (millivolts).

Suppose that the RC time constant of the circuit is much greater than the collection time of the ions, so that the current through R begins to flow as soon as the ions separate in the chamber. Two types of amplifiers are used to make the pulse height proportional to the amount of ionization produced by the particle in the chamber: slow amplifier and fast amplifier. The slow amplifier has a long time constant (RC) compared to the drift time of positive ions, while in fast amplifier, it is made short enough that the potential induced by +ve ions is not of interest. An ionization chamber consists of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer.

The electric field established between the electrodes accelerates the ions produced by the radiation to be collected by the electrodes. This charge is read by the electrometer and can be converted into absorbed dose. When the gas between the electrodes is ionized by the incident ionizing radiation, positive ions and electrons are created under the influence of the electric field. A proportional counter is a modified ionization chamber, one in which a higher voltage is printed, which makes the electric field near the axial cable strong enough to accelerate approaching electrons to such high energies that their collisions with gas molecules cause further ionization.

For example, high-pressure xenon ionization (HPXe) chambers are ideal for use in uncontrolled environments, as their response has been proven to be consistent over wide temperature ranges (20 to 170 °C). This makes the output signal in the ionization chamber a direct current, unlike Geiger-Muller tube which produces a pulse output. The ionization chamber is also used to detect gamma radiation and x-rays, collectively known as photons, and for this, windowless tube is used. The response of an ionization chamber depends to a large extent on voltage applied between outer electrode and center electrode.


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