Answer to Question #8935 Submitted to "Ask the Experts"
Category: Instrumentation and Measurements
The following question was answered by an expert in the appropriate field:
I know a little about ion chambers and have used them. I often read about the materials they are constructed of, such as graphite for the thimble and the use of aluminum for the central electrode. In one of the previous questions you stated that the purpose of the guard ring is to act as a conductor and to prevent leakage. What material is the guard ring most likely made of?
The materials of construction for ionization chambers vary somewhat depending on the intended application. For example, chambers designed to monitor intense radiation fields or to be used in some other physically demanding environment (e.g., high temperature) may use primarily metallic components, including the guard ring, to prolong the lifetime of the chamber. Materials such as aluminum, stainless steel, nickel, and others have been used. Ceramics have sometimes been used as insulators for such chambers.
Many of the ion chambers designed for high-accuracy dosimetry measurements use materials that are nearly tissue equivalent or air equivalent in atomic composition. This often leads to the use of plastics (or, in some cases, graphite) that are near tissue or air equivalent. While metals have been used in some designs, they have the disadvantage that the increased atomic number of metallic constituents compared to plastics or graphite can lead to some possible excess response to photons, especially to low-energy photons (enhanced photoelectric effect) and high-energy photons (enhanced pair production process). When metals are used, aluminum is a common choice because, among the commonly available metals, it has a relatively low atomic number.
The plastics used for electrodes are often made electrically volume conductive, frequently by loading them with graphite during manufacture. In other cases tissue- or air-equivalent nonconducting plastics are used, and the electrode surfaces are made conducting by coating with colloidal graphite. In some cases the conductive coatings have been applied in the form of a very thin metallic coating, aluminum being one of the more common choices.
In a parallel plate chamber, the active surfaces of the disc-shaped collecting electrode and the guard ring lie in the same plane, separated from each other by a narrow insulating gap. The use of conducting plastic or graphite-coated plastic is popular for both collecting electrode and guard ring in such chambers.
For cylindrical and spherical ion chambers, the guard electrode forms a cylindrical conducting shield around the stem end of the collecting electrode, separated from the collecting electrode by a plastic insulator. A second cylindrical insulator separates the guard ring from the outer electrode. For high-accuracy tissue or air dose measurements, it is again desirable to use construction materials that have low atomic numbers, similar to soft tissue or air. Again, conducting plastics and/or graphite-coated electrodes are desirable. The guard electrode may be fabricated by coating the outer cylindrical surface of the insulator with conducting graphite to provide a favorable atomic number. Some such ionization chambers have been fabricated with a thin metallic coating on the outer surface of the insulator.
Depending on the design of the chamber, it may be desirable to have the cylindrical guard ring longer in dimension than the inner insulator, and some chambers have used conducting plastic, machined in the form of a hollow cylindrical guard ring to fit around the inner insulator. Metallic conductors have also been used in designs for some chambers to meet the requirements for sizing and specialized shaping of the guard electrode. While photon interactions in such metallic components may produce some errant response, the magnitude can usually be kept acceptably low (by limiting the atomic number and mass of the metal) and/or be properly assessed.
Ionization chambers intended for routine health physics operational measurements often have less demanding construction requirements than do chambers built for other dosimetry applications, and may use more metal in various instrument parts.
George Chabot, PhD, CHP