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Figure 20 Compensated Ion Chamber with Concentric Cylinders | Up
Instrumentation and Control 2 of 2 | Next
Figure 21 Typical Compensation Curve |
Radiation Detectors
COMPENSATED ION CHAMBER
As the compensating chamber voltage is raised, the measured current will decrease as more of
the current from the working chamber is canceled by the current from the compensating chamber.
Eventually, the voltage becomes large enough so that the two currents cancel. When the currents
cancel, the chamber is said to be 100% compensated, and the measured current is zero. At 100%
compensation, the detector will respond to neutrons alone.
The compensating chamber usually has a slightly larger sensitive volume than the working
chamber. Increasing the compensating current to a value greater than the working chamber
current results in a net negative current.
In this condition, the chamber is said to be
overcompensated. The compensating chamber cancels too much current from the working
chamber, and the meter reads low. In this case, the compensating chamber cancels out all of the
gamma current and some of the neutron current.
Percent compensation of a compensated ion chamber gives the percentage of the gamma rays
which are canceled out. Percent compensation may be calculated based on measured current,
when the detector is exposed to gamma rays only as given in Equation 6-9.
(6-9)
Percent Compensation
1
Imeasured
Ioperating
x 100%
where
Imeasured = measured current (milliamps)
Ioperating = measured current with compensating voltage OFF (milliamps)
If measured current is zero, then percent compensation is 100%. If measured current is positive,
the percent compensation is less than 100%, and the chamber is undercompensated. If the
measured current is negative, the percent compensation is greater than 100%, and the chamber
is overcompensated.
Rev. 0
Page 37
IC-06
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