Video: Screening People for External Contamination: How to Use Hand-held Radiation Survey Equipment

Segment 3 of 4:
Ionization chamber

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Now that we have finished looking at GM detectors, we are going to consider another common gas-filled detector, the ionization chamber. Ion chambers come in many different shapes and sizes. So how do you know that you have an ion chamber? Well, here are a few hints. First, an ion chamber does not have an audio output -- there are no clicks. Second, ion chambers never read out in counts per minute, or cpm. The display will either be in mR/hr or R/hr. And third, everything is housed inside one case. There is no separate probe that has to be connected to the meter with a cable. Here are two different types of ionization chambers. One has a digital readout... And one has an analog readout. While these different ion chambers might not look the same on the outside, they all use the same type of detector on the inside -- an air-filled cylinder approximately 3 inches in diameter. The primary role of ion chambers is to measure the radiation exposure rate due to gamma rays. Ion chambers are usually considered high-end instruments because they are used to measure high exposure rates -- let's say 1 mR/hr and above. An ion chamber has a thin window that is protected by a removable cover. When the cover is removed, the instrument can respond to beta particles. Nevertheless, in most situations, it's probably best to leave the cover on and to forget about its ability to respond to beta particles. After all, a GM detector is a better instrument for detecting beta particles than an ion chamber. In addition, leaving the cover open increases the risk that the window will be broken. Ion chambers do have some disadvantages. First, a typical ion chamber is not sufficiently sensitive to perform measurements at the low exposure rates that you are likely to encounter. Second, ion chambers have a very slow response time. You might have to wait as long as one minute to get an accurate reading. And if you are moving too quickly, you might find yourself in a high-exposure-rate area before the ion chamber has had enough time to respond and let you know what's going on. Finally, an ion chamber will read too high if moisture gets into the chamber. This is a common problem because most ion chambers are open to the atmosphere and the atmosphere contains water vapor. To prevent this from happening, ion chambers are designed so that the air must go through a drying agent, typically silica gel, before it gets into the chamber. One thing you can do to keep your ion chamber in operating condition is to store it in a sealed box, or even a plastic bag that contains silica gel.

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