COMPENSATED ION CHAMBER
The boron coated chamber is referred to as the working chamber; the uncoated chamber is called
the compensating chamber. When exposed to a gamma source, the battery for the working
chamber will set up a current flow that deflects the meter in one direction. The compensating
chamber battery will set up a current flow that deflects the meter in the opposite direction. If
both chambers are identical, and both batteries are of the same voltage, the net current flow is
exactly zero. Therefore, the compensating chamber cancels the current due to gamma rays.
The two chambers of a compensated ion chamber are never truly identical; in fact, they are often
purposely constructed in different shapes. The chambers are normally constructed as concentric
cylinders, as illustrated in Figure 20.
Figure 20 Compensated Ion Chamber with Concentric Cylinders
The use of concentric cylinders has an advantage because both chambers are exposed to nearly
the same radiation field. Even though the chambers are not identical, proper selection of the
operating voltage eliminates the gamma current. Working chamber operating voltage is given
by the manufacturer and is selected to cause operation on the flat portion of the response curve,
where very little recombination occurs. If working chamber voltage is increased to operating
voltage, and compensating voltage is left at zero, the measured current will be due to gammas
only in the working chamber. For this reason, compensating voltage is set while the reactor is
shutdown (a minimum number of neutrons are present).