For the first time absolute photon mass energy-absorption coefficients of air in the energy range 3 keV to 10 keV have been measured with relative standard uncertainties less than 1%, significantly smaller than those of up to 5% assumed hitherto for calculated data. Monochromatized synchrotron radiation was used to measure both the total radiant energy by means of silicon photodiodes calibrated against a cryogenic radiometer and the fraction of radiant energy that is deposited in dry air by means of a free air ionization chamber. The measured ionization charge was converted into energy absorbed in air by calculated effective W values of photons as a function of their energy based on new measurements of the W values in dry air for electron kinetic energies between 1 keV and 7 keV, also presented in this work. The measured absorption coefficients were compared with state-of-the art calculations and found to agree within 0.7% with data calculated earlier by Hubbell at energies above 4 keV but were found to differ by values up to 2.1% at 10 keV from more recent calculations of Seltzer.
For medium energy x-rays produced with tube voltages from 70 to 280 kV, the absorbed dose to water, D(w), has been determined by means of water calorimetry with relative standard uncertainties ranging from 0.45% to 0.98% at 280 and 70 kV. The results were confirmed by Monte Carlo calculations, in which the ratios of D(w) at 5 cm depth in a reference water phantom to the air kerma free in air, K(a), at the same point in space were compared to the corresponding ratios determined experimentally. The general agreement between measurement and calculation was better than 1%. These results confirm earlier investigations in which the absorbed dose to graphite was determined by means of a graphite extrapolation chamber. For the Monte Carlo calculations, an attempt was made to present a complete uncertainty budget, taking into account type B contributions also.
The induction of chromosome aberrations in human lymphocytes irradiated in vitro with X rays generated at a tube voltage of 29 kV was examined to assess the maximum low-dose RBE (RBE(M)) relative to higher-energy X rays or 60Co gamma rays. Since blood was taken from the same male donor whose blood had been used for previous irradiation experiments using widely varying photon energies, the greatest possible accuracy was available for such an estimation of the RBE(M), avoiding the interindividual variations in sensitivity or differences in methodology usually associated with interlaboratory comparisons. The magnitude of the linear coefficient alpha of the linear-quadratic dose-effect relationship obtained for the production of dicentric chromosomes by 29 kV X rays (alpha = 0.0655 +/- 0.0097 Gy(-1)) confirms earlier observations of a strong increase in alpha with decreasing photon energy. Relating this value to previously published values of alpha for the dose-effect curves for dicentrics obtained in our own laboratory, RBE(M) values of 1.6 +/- 0.3 in comparison with weakly filtered 220 kV X rays, 3.0 +/- 0.7 compared to heavily filtered 220 kV X rays, and 6.1 +/- 2.5 compared to 60Co gamma rays have been obtained. These data emphasize that the choice of the reference radiation is of fundamental importance for the RBE(M) obtained. A special survey of the RBE(M) values obtained by different investigators in the narrow quality range from about 30 to 350 kV X rays indicates that the present RBE is in fairly good agreement with previously published findings for the induction of chromosome aberrations or micronuclei in human lymphocytes but differs from recently published findings for neoplastic transformation in a human hybrid cell line.
Thick walled cavity ionization chambers are used by primary standard laboratories as primary air kerma standards in 137Cs and 60Co gamma-rays. Application of the cavity theory requires correction for the effects of photon attenuation and scattering in the chamber walls. For more than a decade there have been intensive discussions about the validity of wall correction factors determined by more traditional extrapolation methods versus those calculated by Monte Carlo methods. For existing primary standards the alternative methods lead to results that differ by up to 50% of the correction itself. This report presents both experimental and theoretical results which strongly support the validity of calculated wall correction factors. Moreover, it is demonstrated that, in selected cases, the application of a linear extrapolation method leads to errors in the determination of the air kerma reaching up to 13%.
A method is described for determining the absorbed dose to graphite formedium energy x-rays (50-300 kV). The experimental arrangement consists of an extrapolation chamber which is part of a cylindrical graphite phantom of 30 cm diameter and 13 cm depth. The method presented is an extension of the so-called two-component model. In this model the absorbed dose to graphite is derived from the absorbed dose to the air of the cavity formed by the measuring volume. Considering separately the contributions of the absorbed dose to air in the cavity from electrons produced in Compton and photoelectric interactions this dose can be converted to the absorbed dose to graphite in the limit of zero plate separation. The extension of the two-component model proposed in this paper consists of taking into account the energy transferred to de-excitation electrons, i.e. Auger electrons, which are produced as a consequence of a photoelectric interaction or a Compton scattering process. For the system considered, these electrons have energies in the range between about 200 eV and 3 keV and hence a range in air at atmospheric pressure of 0.2 mm or less. As the amount of energy transferred to the de-excitation electrons is different per unit mass in air and in graphite, there is a region, about 0.2 mm thick, of disturbed electronic equilibrium at the graphite-to-air interface. By means of the extension proposed, the x-ray tube voltage range over which a graphite extrapolation chamber can be used is lowered from 100 kV in the case of the two-component model down to at least 50 kV.
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