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.
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