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.
The ions will set in motion when an electric field is maintained across the material, resulting in an ionization current. It has been found that approximately 3.5 eV of energy is required to form an ion pair in air. If the incoming particle loses 1 Mev in the chamber, it forms approximately 2.86* 104 ion pairs. The current signal or voltage pulse developed through R is proportional to the number of electrons collected by the electrode.
The amplifiers used can only record pulses of small magnitude (millivolts). 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 time constants that are so long that pulses can accumulate, limiting the maximum counting speed. The fast amplifier has time constants that are 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. 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).
Ionization chambers are preferred for high radiation dose rates because they have no “dead time”, a phenomenon that affects the accuracy of Geiger-Mueller tubes at high dose rates. Primitive total ionization chambers (such as electroscopes used by Rutherford in the early 20th century) are based on the use of an electret, which maintains a charge for an extended period and is discharged by exposure to radiation. Proportional counters are modified ionization chambers, where higher voltage is applied, making electric field near axial cable strong enough to accelerate approaching electrons to such high energies that their collisions with gas molecules cause further ionization. The alpha particle causes ionization inside the chamber, and ejected electrons cause additional secondary ionizations.
The response of an ionization chamber depends to a great extent on its design and construction.
