Visual demonstration of the displacement effect of an ionization chamber within a phantom, at low X-ray energies (< 150 keV)

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The publication of an international code of practice for photon and electron dosimetry by the IAEA has demonstrated discrepancies in the correction factors needed for the displacement effects of ion chambers at low and medium X-ray energies, compared with previous recommendations by ICRU and the HPA. The author describes a method for producing an image of a chamber cavity within a phantom, using both silver bromide and radiation-sensitive film, from which it may be deduced that displacement correction factors are likely, in general, to be less than unity. This is in agreement with several Monte Carlo simulation studies but contrary to IAEA recommendations. Although primarily of qualitative interest, the results suggest displacement correction factors, pu, in the range 0.90 < pu < 1.00 within the quality range 2.2 to 5.2 mm Al HVL.