Ionisation Chamber

Ionisation Chamber

An ionisation chamber, or “ionization chamber”, is an electronic component, which detects radiation by measuring a voltage created by the flow of ions generated by radioactive particles. A gas, which readily forms ions through interaction with radioactive particles, fills the chamber cavity. Two electrodes with a high voltage across them form an electric field, and in the presence of ions in the chamber cause a flow of electric current between them. This flow is usually very small in the femto ampere range and detectable by electronic means.

Electrodes

The chamber is usually cylindrical shaped with a coaxially connected electrode through the centre called anode connected to a positive potential. The wall of the cylinder is usually metal and serves as the cathode connected to a negative potential. A large potential between the electrodes establishes an electric field in every direction axially. The field is strong enough to pull the ion pairs, however not too strong that it creates further ions.

Ion Pairs

When energetic radioactive particles such as alpha, beta, or gamma knock an electron off the outer ring of an atom, which in this case is the gas atom that fills the chamber, it will either lose an electron or gain an electron. When an atom loses an electron, its overall charge becomes positive and therefore the cathode end of the chamber attracts it. However, when an atom gains an electron, its overall charge becomes negative and therefore the anode end of the chamber attracts it.

The flow of these ions therefore constitutes a flow of electron charge proportional to the amount of ions present. Usually, the voltage V required is in the order of hundreds to thousands of volts, whilst the flow of charge within the circuit is in the magnitude of nano amperes, as a result an amplifier is utilised to make this tiny change more apparent.

Detection Voltage

In the circuit shown, the movement of ions betweens the electrode gap, forms a small flow of current through the resistor R completing the circuit.

Through Ohm's law V = I × R we know that when a current I flows through a resistor R we get a potential V across it. In this case, there will be a very small voltage across the resistor, since the flow of current is very small. Since the voltage is small, it requires amplification using high-gain amplifier circuits to a usable form for detection.

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