Question

Identify one function of the guard ring in an ionisation chamber?

Answer 1

The role of guard ring in an ionisation chamber is to reduce the leakage of ionisation current.In all the ionisation chambers an insulating support must be provided between the electrodes.Even a good insulator also leads some small amount of current leakage.But this small amount itself produce false reading since the ionisation current is in the order of .So for an efficient ionisation chamber this leakage current must be very small.So guard rings are employed to reduce leakage of current through indulator.

The image shows a cylindrical ionisation chamber.

The high voltage inner electrode is seperated from the outer negative electrodes (the inner wall of the chamber) by two insulator wraps and a conducting guard ring in the middle of insulators as shown.The guard rind is maintained at the same potential as the central electrode.With this arrangement most of the voltage drop is across the outer insulator.So that resulting leakage does not get in to the central electrode.That is the leakage current will not be shown in the ammeter.

Since the guard ring potential is same as that of central electrode potential charge leakage through the inner insulator can not be possible .Thus gurd ring reduces leakage current.