Ionization Chamber an Overview
What is 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.
Ionization Chamber Principle
The term “ionization chamber” is used to describe those detectors that are based on the principle of excitation or ionization of atoms of the medium through which the incident charged particles pass. The charged particles while passing through matter leave along their paths a chain of ionized or excited atoms which can be detected and counted.
ionization chamber diagram
Most of the detectors measure the ionization produced by the passage of a charged particle through a suitable material. When an electric field is maintained across the material, the ions will be set in motion resulting in ionization current.
Construction of ionization chamber
A simple ionization chamber consists of a metallic 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) as shown in the figure. Thus, wire acts as an anode and cylinder as a cathode.
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 n Ion pairs are produced in the chamber, then ne– electrons shall be attracted towards the anode and ne+ positive ions shall be attracted towards the cathode.
Of course, ne+ shall moves slowly because they are comparatively heavier than ne–. A total charge q = 2 ne will be collected on the electrodes. It has been proved that about 3.5 eV of energy is required to form an Ion pair in the air.
If the incoming particle loses 1 Me v in the chamber, it forms about 2.86*104 ion pairs. According to the figure,
The ionization chamber operates in the region of constant pulse size. In this region, the applied voltage is high enough to prevent the recombination of ions and low enough to prevent gas multiplication.
The current signal or voltage pulse, therefore, developed across 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 are able to record pulses of small magnitude only ( millivolt).
Working and Theory of ionization chamber
Let a track of n ions pairs is formed parallel to the central electrode at a distance Xo. Let V with the voltage across the electrode separate by a distance d. The electrons and positive ions drift in opposite directions in a uniform field E = V/d.
Let us suppose the time constant RC of the circuit is much larger than the collection time of ions so that current through R begins to flow as soon as the ions move apart in the chamber.
Let the ion track was formed at t = 0 and after time t, the change in potential of the positive electrode is given by
For t > te (electron collection time), there is an abrupt decrease in the rate of rising of potential. At t=tp, the time in which the +ve ions reach the negative electrode, the charge of potential becomes – ne/C.
This limit is proportional to n and is independent of where the ions were formed in the chamber.
Two kinds of amplifiers are used to have the height of the pulse proportional to the amount of ionization produced by the particle in the chamber.
(i) Slow amplifier in which the shortest time constant (RC) is chosen long compared with the drift time of the positive ions. This amplifier has the following limitations. The time constants are so long that the pulses can pile up which limits the maximum counting rate.
(ii) Fast amplifier in which time constant (RC) is made short enough so that the induced potential by the +ve ions is not of interest. Thus, the slow linear rise, after the collection of electrons is removed and the height of the recorder pulse is proportional to V.
This defect can be removed by placing a grid at a distance of d/3 from the +ve electrodes. This grid is maintained at a potential of -V/2 so the electric field between the grid and the negative electrode is weaker than the field between the grid and the positive electrode.
When the ionizing particle is confined to the space between grid screens the +ve electrode from the +ve ions and electrons. Due to the drifting of electrons through the grid, change of potential is induced at the +ve electrodes while the +ve ions has no effect as they are screened from the +ve electrons and it attains a maximum value after the collection of all the electrons.
The maximum value pulse height is given by
Ionization chamber advantages and disadvantages
Advantages of Ionization chamber
- Current mode. The ionization chamber has no “dead time” for this reason it is used for high radiation dose rates.
- Simplicity. In an ionization chamber, less expensive and more portable power supplies can be used.
- Neutron Detection. In nuclear reactors, ionization chambers in current mode are often used to detect neutrons and belong to the Neutron Instrumentation System (NIS).
Disadvantages of ionization chamber
- No Charge Amplification. Detectors in the ionization region operate at a low electric field strength, selected such that no gas multiplication takes place.
- Low Density. Gamma rays deposit a significantly lower amount of energy to the detector than other particles.