Sci-Fri PM: Delivery - 04: Quantitative air communication testing of ion chambers for megavoltage dosimetry.
The valid application of the standard correction for air density (PTP in the TG-51 protocol) requires that for a vented ion chamber (basically all reference-class ion chambers) the air cavity does indeed communicate directly with the external environment. However, this assumption is not tested by users, and not universally verified by calibration laboratories. A system has therefore been developed at the National Research Council to test air communication of cylindrical and parallel-plate ion chambers. The systems is based on measurements in a vacuum vessel with a Sr-90 check source; the procedure is simple and quick and can measure ionization currents over the pressure range 0 kPa (atmospheric) to -20kPa (0.8 atm) with an uncertainty better than 0.2%. Investigation of a wide range of chamber types shows that for a coarse check on chamber performance (i.e., that the chamber is vented to atmosphere) measurement at a single polarity is sufficient (total test time less than 15 minutes) but for accurate characterization of the chamber performance, data at both polarities must be acquired. The accuracy of the system means that it can potentially be used to investigate: i) the validity of the pressure correction, and ii) the source of the polarity correction in cylindrical and parallel-plate chambers. The air communication test will be implemented as part of the standard calibration services provided by NRC for external beam radiotherapy.