An EGSnrc investigation of the PTP correction factor for ion chambers in kilovoltage x rays

Abstract

As part of the standard practice for obtaining consistent ion chamber measurements with cavities open to the surrounding atmosphere, the raw measured response is corrected to the response at a reference temperature and pressure using the standard temperature-pressure correction factor (P(TP)). In this study, the EGSnrc Monte Carlo code was used to investigate the validity of the P(TP) correction factor for kilovoltage x rays incident on various geometrically distinct ion chambers. The calculated P(TP)-corrected chamber response deviated by over 2% relative to expected values for a 40 kV spectrum incident on a graphite thimble chamber at an air density typical of Mexico City. The relative deviation from the expected response was much worse for a large spherical graphite chamber, exceeding 16% at an air density of 0.6 kg/m3 (approximately 0.5 atm at 22 degrees C) for the same beam energy. The breakdown of the P(TP) correction factor was also observed for a 26 kV mammography spectrum incident on two mammography chambers. For 60Co beams, the P(TP) correction factor behaved as expected. For day-to-day variations in pressure, only a negligible of the P(TP) correction factor was observed with low x-ray energies. Factors contributing to the breakdown of the P(TP) correction factor at low x-ray energies and large pressure variations, such as the range of electrons, the material of the wall, the chamber dimensions and air-photon interactions, are discussed in depth.