Compensated Ion Chamber with Concentric Cylinders:

The two chambers of a compensated ion chamber are never truly identical; actually, they are frequently purposely constructed in various shapes. The chambers are generally constructed as concentric cylinders, as described in Figure.

Figure: Compensated Ion Chamber with Concentric Cylinders

The use of concentric cylinders has a benefit since both chambers are exposed to nearly the similar radiation field. Even by the chambers is not identical, proper chosen of the operating voltage decreasing the gamma current. Working chamber operating voltage is provided through the manufacturer and is chosen to cause operation on the flat portion of the response curve, where extremely little recombination occurs. The measured current will be because of gammas only in the working chamber if working chamber voltage is increased to operating voltage, and compensating voltage is left at zero. For this purpose, compensating voltage is set although the reactor is shutdown (a minimum number of neutrons are present).

As the compensating chamber voltage is increased, the measured current will decrease as more of the current from the working chamber is canceled through the current from the compensating chamber. Finally, the voltage becomes huge sufficient so which the two currents cancel.  While the currents cancel, the chamber is said to be 100 percent compensated, and the measured current is zero.  At 100 percent compensation and the detector will respond to neutrons alone.

The compensating chamber commonly has a slightly larger sensitive volume than the working chamber. Raising a compensating current to a value greater than the working chamber current results in a net negative current. Within this condition, the chamber is said to be overcompensated. A compensating chamber cancels too much current from the working chamber and the meter reads low.  In that case, the compensating chamber cancels out the entire gamma current and a few of the neutron current.

Percent compensation of a compensated ion chamber provides the percentage of the gamma rays that are canceled out. Percent compensation might be calculated based on measured current, while the detector is exposed to gamma rays only as provided in Equation 6-9.

Percent Compensation =    1 - (I_measured/I_operating) x 100%                                    (6-9)

where

I_measured = measured current (milliamps)

I_operating = measured current along with compensating voltage OFF (milliamps)

 Percent compensation is 100% if measured current is zero. The percent compensation is less than 100%, and the chamber is undercompensated if measured current is positive. The percent compensation is greater than 100%, and the chamber is overcompensated if the measured current is negative.