Quantitative use of film density variations for thickness variation determination by polychromatic X-radiography

Abstract

Abstract Based upon calculation of the attenuation of poly-energetic x-rays through a sample of known composition, derivation of the film contrast factor and estimation of the build-up factor, film densities may be applied for quantitative determination of the inspected sample thickness variation, without the need of imaging a calibration sample during the exposure. The accuracy and reliability of such calculations has been tested using the radiographs of an aluminium step wedge with known thickness variations. This allow calculation of the variation of the build-up factor with aluminium step wedge thickness for relatively small total sample thicknesses and at low x-ray energies. Different exposure at varying exposure time and on different x-ray films were performed in order to assess the film contrast factor and to evaluate the variability of the results on the build-up factor variation with sample thickness. The trend of the build-up factors as a function of sample thickness for different peak x-ray energies shows a minimum at approximately 9–11 mm sample thickness, slowly increasing values for smaller sample thickness values, and rapidly increasing values for larger sample thicknesses. The explanation for the observed minimum is somewhat uncertain, although it can be concluded that the calculation procedure for the linear attenuation coefficient results in a compensation for the beam hardening effect. The calculation procedure for determination of sample thickness variations based on film density measurements is examplified using the results of the build-up factor determination. A 2.0 mm aluminium step thickness difference was derived from film density measurements at an accuracy better than 90%. The accuracy of the results depends mainly on the accuracy of the fit of the film contrast factor as a function of film density and the accuracy of the build-up factor values as a function of the sample thickness.