Ionization Chamber| Radioactivity Measurement

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Ionisation Chamber:

• Simplest of all gas chambers 
• Used for measuring xrays, gamma rays and beta particles 
• Used to represent all those chambers that use direct ionisation — through the application of electric field 
• It only uses discrete charges created by each interaction between the incident radiation and gas and does not involve gas multiplication mechanism involved by other gas chambers such as gm counter and proportional counter.
• Ionisation chambers have a good uniform response to radiationa over wide range of energies and other preferred means of measuring high levels of gamma radiation 
• Widely used in nuclear power plants, research labs, radiography radio, biology and environment monitoring principle of operation.
• It measure the charge from the number of ion pairs created within the gas caused by the incident radiation.
• It has a gas filled chamber with two electrodes, anode and cathode — the electrodes maybe in the form of parallel plates or in the cylindrical arrangement with a coaxially located internal anode wire. A voltage potential is applied between the two and the electrons in the upper part of the electrodes to create electric field in the fill gas. When the gas between electrodes is ionised by incident ionising radiation ion pairs are created. As a result, positive ions and dissociated electrons move to the electrodes of opposite polarity. The influence of electric field creates ionisation current which is measured by an electron meter circuit.

• The electron meter must be able to measure very small current which is in the range of Femto amperes to pico amp depending upon the chamber chamber design, radiation dose and applied voltage. An ion pair created deposits or removes electron to or from the electrode such that the accumulated charge is proportional to the no of ion pairs created, and hence the radiation dose.
• This continual generation of charge produces an ionisation current which is the measure of total ionisation dose entering the chamber. 
• Disadvantages
  

• The chamber cannot discriminate between energy types
• Chamber cannot cannot produce energy spectrum of radiation 
• The electric field also enables the device to work continuously by mopping up the electrons which prevents saturation of fill gas where no more ions could be collected and by preventing recombination ion pairs which would diminish the ion current ... this mode of operation is current mode. Meaning output is continuous current unlike gm tube or prop counter referring to ion pair collection graph.
• In there operating regions, collection of ion pairs is constant over a wide range of applied voltages as due to its low electric field strength, ion chamber does not have any multiplication effect unlike hm gm tube and prop counter, where the secondary electrons and multiple avalanches greatly amplify original ion current.