Radiation DetectorsCOMPENSATED ION CHAMBERAs the compensating chamber voltage is raised, the measured current will decrease as more ofthe 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 currentscancel, 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 workingchamber. Increasing the compensating current to a value greater than the working chambercurrent results in a net negative current. In this condition, the chamber is said to beovercompensated. The compensating chamber cancels too much current from the workingchamber, and the meter reads low. In this case, the compensating chamber cancels out all of thegamma current and some of the neutron current.Percent compensation of a compensated ion chamber gives the percentage of the gamma rayswhich 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 Compensation1ImeasuredIoperatingx 100%whereImeasured = 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 themeasured current is negative, the percent compensation is greater than 100%, and the chamberis overcompensated.Rev. 0 Page 37 IC-06
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