Bent and flat highly oriented pyrolytic graphite crystals as small bragg angle monochromators in thin‐specimen energy‐dispersive XRF analysis

Abstract

AbstractA monochromatic high‐intensity x‐ray beam, obtained by Bragg reflection of the strongest characteristic tube line under small angles on single crystals, can be used as an EDXRF excitation source. A systematic selection procedure and search of 55 single crystals suitable as EDXRF Bragg reflectors was made and highly oriented pyrolytic graphite (HOPG) was found to be a superior candidate for the characteristic Kα line of an Mo tube within the range of low‐order reflections, yielding the highest intensities. Meridionally and sagitally curved HOPGs were studied in a flexible experimental set‐up and the reflected beam intensity and profile were compared with the corresponding data obtained from flat HOPGs with different mosaic spreads. The relationships between collimation, crystal curvature and mosaic spread were evaluated. The experimental angular beam acceptance and the secondary extinction effects dependent on the mosaic spread of the HOPG were found to be in line with theoretical expectations. Measured in similar configurations as required for realistic EDXRF applications, the sagitally bent HOPG increases the reflected beam intensity up to 77% whereas the meridionally bent HOPG yields an intensity gain of only up to 28% compared with the flat crystal with the same mosaic spread. The detection limits for the corresponding thin‐filter EDXRF analysis are presented.