Dependence of XRF intensity on the tilt of the propagation plane

Abstract

A particular parameterization of both the incident and the take-off angles in the narrow beam model of the x-ray fluorescence (XRF) intensity, is introduced. The angles are related to the tilt α of the propagation plane (the plane defined by the incident and emerging beam's directions) and the XRF intensity is studied as a function of α. An interesting geometrical property of XRF is revealed. The primary XRF intensity remains invariant when α changes whereas the secondary and tertiary XRF intensities show a monotonic decrease when α increases. They vanish for the maximum tilt of α = π4 2. The different behaviours of the primary intensity on the one hand and the higher orders of XRF on the other allow the isolation of the primary emission. This would not otherwise be possible because the photons under the same spectral line come from all the emission orders. As they have the same energy, they cannot be resolved by the detection system. The theoretical predictions were confirmed experimentally. Measurements were made with a specially built device. Because of the difftculties of measuring near-horizontal emission, the limit-angle intensity is determined by extrapolation from intensity measurements at several α. As an example, primary emission of two NBS stainless-steel standards was obtained. Isolation of the secondary emission is also possible where higher orders are not involved. The fundamentals of this phenomenon may be better understood with the help of a Monte Carlo simulation of the XRF emission from finite slabs, embedded in the infinite target. The simulation shows the strong dependence of the layer emission on α. This result makes it possible to suggest that the angle a may be used as α ‘tuner’ of radiation coming from a given depth. Some practical applications of this geometrical property of XRF intensity are discussed. The intensity change due to a slight tilt of the propagation plane (i.e. a sample improperly placed in the sample holder), is found to besmall. Also, two spectrochemical methods of analysis benefiting from the composition dependence of the reduced intensity and the selective emission from different depths applied to the thickness gauge of thin samples are con-sidered.