Application Progress of Pyrolytic Graphite Crystals in X-Ray Fluorescence Spectrometer
Graphite crystals with excellent chemical inertness,high melting point,strong elasticity,and unique electronic properties are widely used in many fields.Pyrolytic graphite is increasingly used in XRF technology due to its excellent high integral reflectivity for X-rays.Pyrolytic graphite is a synthetic graphite material with a layered structure.Annealing close to 3 000 ℃ can adjust the material structure.The structural changes are enhanced,while the annealing is accompanied by deformation.Different combinations of annealing temperatures and types of deformation result in the production of pyrolytic graphite with different structures;there are mainly two types of pyrolytic graphite:highly oriented pyrolytic graphite(HOPG)and highly annealed pyrolytic graphite(HAPG),respectively.HOPG crystals,typically having a mosaic spread in the range of 0.3 to 3°,offer a higher integral reflectivity among 2 keV to tens of keV,which is at least an order of magnitude higher than the reflectivity of all other crystals for X-rays,but moderate resolution.The HOPG energy resolution is mainly affected by mosaic spread and intrinsic widths of the Bragg reflections;larger mosaic spread results in significantly lower energy resolution.HAPG crystals are a modified material similar to HOPG crystals,with a lower mosaic spread of 0.05 to 0.l°and a higher resolution but a lower integral reflectivity.Its mosaic spread is about 1/5 compared with HOPG crystals of the same thickness.Monolayer HAPG crystals can be deposited on the substrate and bent into small radii and almost any shape without significantly reducing the energy resolution.Graphite crystals are mainly used as monochromators in the field of X-ray fluorescence,which can reduce high background caused by white light,fluorescence,and stray light in Von Hamos spectrometers and EDXRF instruments,the Von Hamos spectrometer with the focusing of pyrolytic graphite crystals can obtain a better integral reflectivity and energy resolution;which improved the sensitivity and detection limit of the analysis.It also performs well in studying fine structures,measurements of radioactive elements,and mammography.This paper mainly reviewed the research and applications of pyrolytic graphite in X-ray fluorescence spectrometers in the last two decades.
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