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Determination of the mass-energy absorption coefficients of air in the hard X-ray region


In collaboration with Working Group 7.13 "X-ray Radiometry", comparison measurements of the free-air ionisation chamber PK100 were carried out in several beam radiation times on the BAMline at the electron storage ring BESSY II in Berlin by means of the cryogenic radiometer SYRES I which had been further developed by WG 7.13 for the hard X-ray range. To this end, the radiation power of the narrow-band photon radiation (relative bandwidth in the order of 10-3) in the energy range between 30 keV and 93 keV was determined by means of the cryogenic radiometer SYRES I and, at the same time, the ionisation current of the PK100 which, in the beam path, is mounted before the cryogenic radiometer. For smaller photon energies between 10 keV and 30 keV, the radiation power had to be determined with the aid of a different measuring arrangement due to the larger air absorption in this spectral range. In this measuring set-up, the PK100 and several semi-conductor photodiodes - whose spectral sensitivity had previously been determined by calibration with the cryogenic radiometer - were arranged on a displacement stage in such a way that the latter were located at the same distance from the radiation output window as the centre of the detection volume inside the PK100 and could be irradiated alternately with the PK100 by lateral displacement. The measurements with this arrangement for indirect comparison to the cryogenic radiometer were also carried out for higher photon energies of up to 80 keV to allow the application of detector models to verify the absorptance of the radiometer receiver as calculated by means of Monte Carlo simulations using GEANT4 and PENELOPE in the energy range below the gold K edge, in which the gold-and-copper-made cavity absorber is partly transmitted. The material layer thicknesses used in the simulations were determined on the basis of transmission measurements which were carried out on the cavity absorber specifically for this purpose.

From the measurement results for the radiant power and the ionisation current of the PK100, it is possible to determine - from the correction factors determined in connection with the establishment as a primary standard for air kerma and from the known dimensions of the measuring electrodes used - the mass energy-absorption coefficient. The experimental values obtained in this way are shown in the figure in which also corrected results from previous experiments for photon energies below 10 keV are contained [1]. The deviation from common literature values in the order of between 2% and 3%, which was detected in these experiments, seems to apply equally to higher photon energies.

Figure : Experimental results with regard to the mass energy-absorption coefficient of X-ray photons in air (red and green circles: results of the recent experiments; triangles: results from Ref. [1]).


  1. Büermann, L.; Großwendt, B.; Kramer, H.-M.; Selbach, H.-J.; Gerlach, M.; Hoffmann, M.; Krumrey, M.:
    Measurement of the X-ray mass energy-absorption coefficient of air using 3-10 keV synchroton radiation, Physics in Medicine and Biology,
    51 (2006), 20, 5125 - 5150