Answer to Question #8149 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
You do not specify the type of ionization chamber you are concerned with. Among the most common types are the cylindrical chamber that contains a cylindrical central electrode along the longitudinal central axis of the chamber and the parallel plate chamber in which the two major electrodes are often disc-shaped and parallel to each other. The orientation and functioning of the guard rings (or guard electrodes) in these two designs are somewhat different. The guard rings play a major role in reducing the leakage of extraneous charge to the collecting electrode.
In the cylindrical design the central electrode, often the high-voltage electrode, is separated from the outer electrode, represented by the inner wall of the ionization chamber, by insulating material, often a plastic material. Because the ionization chamber is often used to measure quite small currents induced by the radiation field, small amounts of charge leakage through the insulator may contribute to the measured current and provide false readings. Even very good insulators may not have sufficient electrical resistance to reduce such leakage to a negligible level. To get around this problem such ionization chambers are often designed with a guard electrode, which is usually a cylindrical conductor that wraps around the insulator that is in contact with the inner central electrode. A second insulator wraps around the outside of the guard electrode and separates the guard from the outer electrode. The guard electrode is maintained at the same potential as the central electrode. With this arrangement the major voltage drop across the insulator is across the outer insulator between the outer electrode and the guard ring so that any charge leaking in does not get to the central electrode. Since the potentials of the guard electrode and the central electrode are the same, there is virtually no driving force to induce charge leakage across the inner insulator.
In the parallel plate chamber the charge-collecting electrode is surrounded by an annular ring. The annular ring represents the guard ring (or guard electrode) and is separated from the collecting electrode by a narrow insulating gap, and the applied voltage to the guard ring is the same as that to the collecting electrode. The other parallel plate electrode has a diameter at least as great as the outer diameter of the guard electrode. With this geometry and voltage arrangement the guard electrode serves two major purposes, although they are directly linked. First, it ensures that the electric field lines near the edge of the collecting electrode remain straight, perpendicular to both the collecting electrode and the second electrode so that the collecting volume is accurately defined by the area of the collecting electrode and the electrode separation. (If no guard electrode is used, the field lines near the edge of the collecting electrode will tend to bow out, resulting in a poorly defined charge collecting volume.) Secondly, maintaining the same voltage on the collecting electrode and the guard ring minimizes the extent of charge leakage from the volume outside of the collecting volume because there is no significant driving potential to promote this additional charge collection by the collecting electrode in preference to collection by the guard electrode.
George Chabot, PhD, CHP