Ionisation Chamber Principle



The principle of the ionisation chamber is very simple. The chamber shape could be simply two metal plates in parallel, or a simple cylindrical can with an electrode through the centre. One electrode is called an Anode (+ve) while the other is called the Cathode (-ve). Normally a large voltage V DC is maintained between the electrodes to create an electric charge. The charge between the electrodes should be enough to pull the ions but not too much that it creates further ionisation.



When ionising radiation interacts with air, it produces an ion pair which consists of an electron and a positively charged atom. The process of ionisation involves an energetic particle such as alpha, beta, or gamma, knocking an electron off the outer ring of an atom, and thereby making the atom more positive. An atom can either gain an electron or lose an electron from it's outer ring, at which point it is called an ion.

In the presence of charge created by a V DC current, the positive particles will migrate towards the negative terminal, and the negative particles will be attracted towards the positive terminal. The flow of these particles therefore constitutes a flow of electron charge proportional to the amount of ions present. Typically the Voltage V DC required is in the magnitude of hundreds to thousands of volts, whilst the flow of charge within the circuit is in the magnitude of nano amps, as a result an amplifier is needed for better observation.

The cylindrical tube seen above is typically filled with air. As ions reach their respective electrodes, a small amount of current will flow from the battery and through the resistor R making a circuit. Through ohm's law (V=IR) we know that when a current flows through a resistor we get a potential across it, in this case there will be a very small voltage across the resistor, since the flow of current is very small. The voltage can then be amplified using an amplifier and measured.

The current levels detected are very small. Using basic mathematics one can calculate the current produced by a single electron where an electron has the charge of 1.6 × 10 ^-19 coulombs.

Current is defined as charge per second.
I = q / t    where I is the current in amps, q is the charge, and t is time.

if we set t = 1 second and substitute for q the charge on an electron.

I = 1.6 × 10 ^-19 amperes which is the current produced by one electron.

Luckily one alpha particle can create thousands ions through multiple collisions, which can help increase the overall charge and therefore increase the electron flow to the femto amp range.

The down side is that any circuit that detects currents in the femto range, will also most likely detect other electromagnetic noise such as static, as well as leakage currents, which means that the circuit will have to be built with a lot of insulation.

Perhaps in the future chip manufacturers such as Texas Instruments will produce a new chip that is an all-in-one package containing the ionisation chamber as well as the high gain amplifiers.


Electronic Engineering
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Author: Peter J. Vis