The use of the characteristics of highly orientated pyrolytic graphite post-diffraction monochromators to remove Cu Kβ and W Lα radiation

Full text: Highly orientated pyrolytic graphite (HOPG) post-diffraction monochromators offer significant advantages over the use of a Kβ filter in the case of copper radiation, the most significant of which is the suppression of fluorescent radiation at the detector as well as the virtual suppression of Kβ. The bandpass of a HOPG crystal is considerably greater than the Kα packet and hence a small proportion of Kβ will be passed by the monochromator. As a copper xray tube ages it is likely that tungsten from the filament will be deposited on the target. The tungsten will fluoresce as the electrons strike the target. W Lα is much closer in wavelength to Cu Kα than is Cu Kβ and a very significant proportion of W Lα will be passed by the HOPG monochromator and focused onto the detector face. The spatial resolution of HOPG and the dimensions of the post-diffraction focusing circle means that Cu Kα, Cu Kβ and W Lα will focus to different areas of the detector face. A slit or an edge positioned correctly is usually sufficient to greatly suppress Cu Kβ and W Lα. An aging copper tube with a tungsten deposit will usually show a significant fall in output. If the post monochromator slit is well positioned, then the cause of this fall off in intensity is not apparent. While W Lα is sufficiently different from the characteristic lines of cobalt and molybdenum and will not cause a problem if these target types are used, there is still Kβ radiation passed by the monochromator. A correctly positioned mask will ensure the Kβ contribution is greatly suppressed. It is recommended that a post-monochromator mask be installed if one is not already present and that the scatter slit and post-monochromator mask be removed occasionally to ensure that both are positioned correctly. Copyright (1999) Australian X-ray Analytical Association Inc