An ionization Chamber is used for detecting mainly the α and β rays and to compare the activities of the source emitting these rays.
Construction of Ionization Chamber
Basically, it consists of a chamber filled with a gas like air or argon at normal pressure it is fitted with a pair of electrodes E1 and E2 between which a constant p.d. is maintained. It also carries a window W made of mica, 0.002 mm thick and coated with graphite to make it conducting.
When the rays to be detected enter the chamber through the window, they ionize the gas between the electrodes. The positive ions & the electron produced in the chamber move in opposite directions towards the electrodes, so that an ionization current ( ≈ 10^-12 A) flows in the external circuit.
This current is measured by measuring the p.d. established by it in high resistance R by means of an electrometer. The p.d. across the electrodes is so selected that all the ions collect at that electrode & there is no secondary ionization.
Under the condition, the ionization current is proportional to the number of pairs of ions & hence to the number of particles entering the chamber. Therefore, the activities of the sources can be compared by comparing their ionization current. i.e.
“more ionization current means more activity”
An ionisation chamber is less sensetive to β-particles in comparison to α-particles because β-particle produce less ionisation.
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