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Radiation Protection Dosimetry 54:213-215 (1994)
© 1994 Oxford University Press
On the Use of Conversion Coefficients from Air Kerma to the ICRU Quantities for Low Energy X Ray Spectra
For X ray spectra with generating potenials below about 30 kV the predominant contribution to the magnitude of the air kerma is from the low energy part of the photon fluence spectrum. Conversely, the value of personal or ambient dose equivalent for a depth of 10 mm is predominantly determined by the high energy part of the spectrum. Consequently, the value of the conversion coefficient between these two quantities critically depends on the exact shape of the photon fluence spectrum incident at the point of test. The use of standard conversion coefficients may lead to substantially wrong calibrations, as the spectrum at the point of test is not necessarily identical to the spectrum used for the determination of the conversion coefficient. The impact of different spectral shapes was analysed by means of Monte Carlo simulations. The most dramatic effects ere found for an X radiation with 10 kV generating potential and 1 mm Be inherent filtration. The addition of 1 m of air path between the focus and the point of test resulted in an increase of 100% in the value of the conversion coefficient applicable to the spectrum at the point of test. Even for a fixed distance of 1 m between focal spot and point of test the conversion coefficient varies by 5.5%, due to the normal variation of the density of air as a function of temperature and pressure. In general, additional filtration leads to an increase of the value of the conversion coefficient. However, the presence of materials such as tungsten, with an L-absorption edge in the energy region of interest, may also lead to lower values of the conversion coefficient. This effect may be important because of different anode angles of the X ray tube, anode roughening and the anode material which has been evaporated on the the Be window. The effects of spectral impurities are also examined for selected conditions for the production of fluorescence radiations. An attempt is made to derive some recommendation for the safe use of conversion coefficients. The findings are discussed in terms of the radiation protection relevance of Hp(10)f and Hp(0.07) for different photon spectra.