The combined effect of acceleration voltage and incident beam orientation on the characteristic X-ray production in thin crystals

A systematic experimental study has been carried out to determine the combined effect of acceleration voltage and incident beam orientation on the characteristic X-ray production in thin crystals. For MgAl2O4 it has been shown that the orientation dependence undergoes a reversal in character above a particular voltage, which is referred to now as the 'inversion' voltage. This inversion voltage has been experimentally determined to be approx. 270 kV for MgAl2O4 compounds with a spinel structure and is in agreement with theoretical predictions based on a highly localized scattering model for characteristic X-ray production in thin crystals. Further, in combination with theoretical calculations, this 'inversion voltage' behaviour has been shown to be different from the conventional critical voltage effect. From the microanalysis point of view, it has been experimentally shown that in order to obtain an analysis independent of the incident beam orientation or the acceleration voltage, it is essential to systematically tilt the crystal to an orientation at which no lower-order Bragg reflections are excited. (author)