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Semitransparent HOPG monochromators are being investigated with an emphasis on their potential applications in X-rayimaging systems. HOPG plates and films, 5-100 jtm thick and up to 14 cm2 in area, were tested. Scanning techniques wereused to measure the profiles of local reflectivity and transparency at different monochromator thicknesses. Mosaic spread,AU), down to 3.6 was recorded. HOPG film areas with average Aa=5 and 6' were equivalent to 4.5 and 10 cm2 respectively.The echelon monochromator arrangement produced a sharp increase in reflectivity leading to the possibility of primary beam sweeping. Results to date are very promising for future applications in fields such as medical diagnostics and surgical treatments monitored using digital X-ray cameras.Keywords: Semitransparent monochromator, HOPG, X-Ray imaging 1. INTRODUCTION In biomedical applications, decreasing dose rates is one of the basic goals in the design of penetrating radiation systems'. Producing high contrast images using near K-jump absorption2 is another important criteria. As an example, duringangiography it is especially desirable to obtain crisp and clear images of vessels without shadows introduced by otherbiological tissues. Both problems are directly addressed by the development of effective systems capable of converting

The use of bent crystals with high integrated reflectivity in focusing crystal spectrometers (Johann and von Hamos schemes) is considered. It is shown that in a von Hamos scheme mosaic focusing takes place. Thus a mosaic crystal simultaneously provides high spectral resolution and high efficiency. Expressions for the mosaic focusing are obtained. Focusing mica and graphite crystal von Hamos spectrometers (radius of crystal curvature is 20 mm) are investigated: spectral and spatial resolution and absolute efficiency are measured in a spectral range of 2 - 2.6 angstroms using laser-produced plasma and iron isotope x-ray sources. The mica crystal spectrometer showed high spatial (up to 10 micrometers ) and spectral ((lambda) /(delta) (lambda) approximately 1000) resolution, whereas the graphite spectrometer showed very high efficiency (30 - 70 times higher than the mica crystal) and moderate spectral resolution ((lambda) /(delta) (lambda) approximately 500 - 750). In the latter case mosaic focusing is observed: spectral resolution is 10 - 15 times higher than spectral resolution determined by the mosaic spread of the crystal ((lambda) /(delta) (lambda) approximately 50). The results allow one to estimate a maximum efficiency for focusing crystal spectrometers. Prospects for using the von Hamos spectrometers for x-ray spectroscopy and x-ray fluorescence are considered.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.