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Trevor Dowling, David King, Mark Henderson and John White
The role of the Bragg crystal analyser is to isolate selected wavelengths from a polychromatic x-ray beam reflected from a sample (pink beam), so that intensity measurements of those wavelengths can be measured. The Bragg crystal spectrometer uses the phenomenon of Bragg reflection from flat crystals. These crystals may be graphite, silicon or sapphire. If polychromatic x-ray radiation irradiates a flat crystal at an incident angle Θ only radiation with the wavelength λ is reflected with λ fulfilling the Bragg equation:
nλ = 2d sinΘ
Here d is the layer distance of crystal layers parallel to the crystal surface and n the reflection order. Using this principle it is possible to get x-ray spectra of a surface by rotating the crystal with a constant angular velocity and detect the reflected radiation with an avalanche photodiode. Figures 1, 2 and 3 below show respectively: a molybdenum rotor bearing five graphite crystals; the position of the Bragg rotor and the avalanche photodiode and a photograph of the avalanche photodiode mounted onto the rotor housing.
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