WDS Spectrometers

Wavelength dispersive spectrometers measure x-ray intensity as a function of wavelength. This is done by passing radiation emanating from the specimen through an analyzing diffraction crystal mounted on a 2q goniometer. By Bragg’s Law, the angle between the sample and detector yields the wavelength of the radiation.

(6.1)

where

d is the d-spacing of the analyzing crystal

q 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 6.1 shows a simplified schematic of the WDS 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 rejected. The x-ray beam is usually collimated before and after the analyzing crystal.

The goniometer angle for a characteristic line is determined by the d-spacing of the analyzing crystal. Different analyzing crystals are meant for different ranges of wavelength, so WDS spectrometers usually include a set of analyzing crystals to cover the wavelengths of the characteristic lines of all measured elements up to the expected minimum wavelength of the primary radiation.

WDS spectrometers are usually larger and more expensive than other spectrometers. Because the analyzing crystal d-spacing determines wavelength sensitivity, they are usually more sensitive than other spectrometers. To overcome losses in x-ray optics of the WDS spectrometers and to maximize primary radiation intensity, x-ray tubes are usually employed. The sample is usually held under vacuum to reduce contamination and avoid absorption of light element characteristic radiation in air.

 

Figure 6.1 Schematic of a simplified wavelength dispersive x-ray spectrometer.

Qualitative analysis requires a plot of x-ray intensity as a function of wavelength. In older WDS spectrometers this was accomplished with a ratemeter attached to a chart recorder. The qualitative scan would be acquired by plotting with the chart recorder as the spectrometer gomiometer was scanned over a range of angles. In contemporary computer-controlled spectrometers, qualitative scans are acquired by stepping the gomiometer through a series of angles separated by some small step size angle. The qualitative analysis software then displays the qualitative scan by joining the individually acquired counts.

Quantitative analysis requires the intensity of each characteristic line of the elements of the unknown specimen. Because of the excellent angle resolution of a WDS spectrometer, and because peak profiles are determined by the optics of the spectrometer and not the sample itself, the intensity of such a line is usually proportional to its height. It then becomes a simple matter of measuring the counts at a single angle for each characteristic line of the specimen. Counts should also be measured at angles away from characteristic line peaks to account for background. Sometimes account is also required for the overlap of a measured characteristic line with a line of another element in the specimen.

XRFWIN for WDS has tools for performing both qualitative and quantitative analysis with a WDS spectrometer. The WDS spectrometer interface is implemented through an instrument driver. The instrument driver appropriate to the utilized spectrometer make and model is fully integrated into the software so instrument control and analysis are seamless to the operator. XRFWIN Basic for WDS includes a spectrometer driver for a generic instrument. Instrument settings and properties important in analysis are defined for the instrument, and the raw data is imported or pasted into XRFWIN Basic for WDS to be analyzed. XRFWIN Light for WDS includes the instrument driver and tools for managing the spectrometer, but does not include any of the analysis features of the full version of XRFWIN for WDS.


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