Calculations of ion chamber displacement effect corrections for medium-energy x-ray dosimetry

This paper investigates the displacement effect corrections for ionization chambers calibrated in air in terms of air kerma and used at a depth in a phantom irradiated by medium-energy x-rat beams for tube potentials between 100 kV and 300 kV. The EGS4 (electron gamma shower version 4) Monte Carlo code system has been used to simulate the coupled transport of photons and electrons. The chamber displacement correction factor, pdis, has been calculated both using a simple photon attenuation and scattering method and by direct Monte Carlo calculation of the ration of the water kerma at a depth in a phantom to the water kerma in a low-density (the same density as that of air) water cavity with its centre placed at the same depth in the phantom irradiated by medium-energy x-ray beams. Good agreement has been achieved between the pdis factors obtained using the simple method and those obtained by the direct Monte Carlo simulation. In contrast to the values recommended by the IAEA code of practice, which exceed unity by up to 10%, our pdis values for a stemless NE2571 chamber vary between 0.994 and 1.001 for the same energy range and for a stemless NE2561 chamber between 0.974 and 0.993. The beam quality correction factor and stem correction factor have been investigated in separate works. The ratios of the overall correction factors calculated in this work for an NE2571 chamber to that for an NE2561 chamber are consistent with the recent experimental results.