X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

crystal mounted on a 2θ goniometer. By Bragg’s Law, the angle between the sample and
detector yields the wavelength of the radiation:
2dsinθ= nλ;
(IV.1)
where:
d is the d-spacing of the analyzing crystal,
θ is half the angle between the detector and the sample,
n is the order of diffraction.
The analyzing crystal must be oriented so that the crystal diffraction plane is directed in
the appropriate direction. Figure IV.1 shows a simplified schematic of the WD-XRF
spectrometer. A scintillation or flow-proportional detector usually measures the fluoresced
radiation. The heights of the resulting pulses are proportional to energy so using a pulse
height analyzer (PHA), scattered or undesired diffraction-order X-rays can be ejected. The Xray
beam is usually collimated before and after the analyzing crystal.
Each of the components showed in the Figure IV.1 were be described in the following
sections.
II.1 Collimator Masks
Figure IV.1: Schematic description of WD-XRF principle.
The collimator masks are situated between the sample and collimator and serve the
purpose of cutting out the radiation coming from the edge of the cup aperture (Figure IV.2).
The size of the mask is generally adapted to suit of the cup aperture being used.
The masks perform one of the two functions: background reduction and improved
fluorescence (Figure IV.3).