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WDS

(Wavelength Dispersive Spectroscopy)

Wavelength dispersive spectroscopy is what gives the electron microprobe its analytical power.  In wavelength dispersive spectroscopy, the wave properties of x-rays are used to identify and quantify the concentrations of the elements present in a sample by making us of the Bragg equation:

nλ = 2dsinØ

where

    n= the order of diffraction (1,2,3...)

    λ = x-ray wavelength

    d = interplaner d-spacing of diffracting crystal

    Ø = angle of incidence of x-ray on diffracting crystal

X-rays leave the sample in all directions and at all angles, but only those X-rays leaving the sample at 40 degrees (the Take Off Angle) can enter the wavelength spectrometer.

Inside the spectrometer is a diffracting crystal with a constant and known interplaner d-spacing (d). For a particular angle of incidence (Ø) only the X-ray wavelength (λ) that satisfies the Bragg equation will be diffracted to the detector.

Because the angle at which the x-rays strike the defracting crystal determines which element's x-rays will be defracted to the detector, changing the angle of incidence changes the element detected.

Example:

Illustration showing Rowland Circle, diffracting crustal and detector geometry, with 'close-up' of diffracting crystal illustrating  the Bragg equation.
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