Feasibility study of in-house second-channel calibration of dual-channel electrometers using a battery-powered...

An ionization chamber and electrometer allow measurement of the absorbed dose to water. A sensitivity comparison between electrometers is essential for quality control, and an efficient method is available to accurately measure the electrometer sensitivity coefficient without using a linear accelerator (linac). Although dual- circuit electrometers are becoming increasingly common, no calculation method for the sensitivity coefficient of their second-circuit is available. Hence, we propose a method for calculating this sensitivity coefficient using the first-circuit as the reference and evaluate its accuracy.

Using the first-circuit of a RAMTEC pro electrometer as a reference, the RAMTEC duo and SuperMAX electrometers were connected as test units to the simple yet accurate Japanese-patented SCG002 current source powered by a dry cell battery. Sensitivity ratio r_elec​ was calculated from the average of three charge measurements using RAMTEC Pro. This ratio was multiplied by the calibration coefficient of the first- circuit to obtain the sensitivity coefficient of the second-circuit. The accuracy was obtained from the relative error of each electrometer based on the calibration coefficient (k_elec) provided by a standards laboratory.

The sensitivity coefficient of the second-circuit of RAMTEC pro was 1.0004 (relative error, +0.030%). For RAMTEC duo, the first- and second-circuit coefficients were 1.0014 and 1.0013, respectively (relative errors, +0.080% and +0.070%). For SuperMAX, the coefficients were 0.9986 and 0.9983 (relative errors, 0.0% and -0.050%) for the first and second circuits, respectively. Thus, the proposed method provided accurate measurements.

We accurately determine the sensitivity coefficient of the second-circuit in a dual-circuit electrometer using the first-circuit of the same or another electrometer as the reference. If the electrometer performance is verified, the coefficient k_elec of the first-circuit is likely applicable to the second-circuit. This method may reduce the costs associated with electrometer calibration in clinical settings.