CORRECTION FACTORS FOR THE NMI FREE-AIR IONIZATION-CHAMBER FOR MEDIUM-ENERGY X-RAYS CALCULATED WITH THE MONTE-CARLO METHOD

A new method is described for the determination of x-ray quality depen dent correction factors for free-air ionization chambers. The method i s based on weighting correction factors for mono-energetic photons, wh ich are calculated using the Monte Carlo method, with measured air ker ma spectra. With this method, correction factors for electron loss, sc atter inside the chamber and transmission through the diaphragm and fr ont wall have been calculated for the NMi free-air chamber for medium- energy x-rays for a wide range of x-ray qualities in use at NMi. The n ewly obtained correction factors were compared with the values in use at present, which are based on interpolation of experimental data for a specific set of x-ray qualities. For x-ray qualities which are simil ar to this specific set, the agreement between the correction factors determined with the new method and those based on the experimental dat a is better than 0.1%, except for heavily filtered x-rays generated at 250 kV. For x-ray qualities dissimilar to the specific set, differenc es up to 0.4% exist, which can be explained by uncertainties in the in terpolation procedure of the experimental data Since the new method do es not depend on experimental data for a specific set of x-ray qualiti es, the new method allows for a more flexible use of the free-air cham ber as a primary standard for air kerma for any x-ray quality in the m edium-energy x-ray range.