HOPG is traditionally considered as a crystal aimed for applications requiring high brightness and moderate spectral resolution. However application of HOPG in von Hamos geometry (Fig.1) allows escaping this restriction.
Fig.1. The von Hamos X-ray spectrometer consists of a cylindrically bent crystal, a position sensitive detector located on the cylinder axis and a radiation source on this axis as well. |
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In following geometry mosaic focusing takes place (Fig.2) and mosaicity does not limit any more the spectral resolution.
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Fig.2. Mosaic focusing arises from the fact that X-rays from a monochromatic point source reflected on a mosaic crystal converge to a point of equal distance from the crystal as the source before diverging away. So in the symmetric geometry mosaic focusing enhances the intensity in the image plane as well as the spectral resolution. |
The energy smearing is determined by geometrical aberration, source size and the Darwin width of small crystalites (intrinsic broadening). And though geometrical aberration (mainly flat focusing errors and errors due to X-ray penetration into mosaic crystal) are still influenced by crystal mosaicity, the high energy resolution could be observed (Fig.3).
Fig.3. Smearing in the image plane: contribution of different factors calculated for 8 keV as a function of source to crystal and crystal to detector distance F [Ref.10]. |
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Difficulties in classical hot-press technology of bent HOPG crystals cause high price and low geometrical precision of the crystals – problems, which rapidly increase when the radii decrease down 220 mm.
Optigraph technology of HOPG/ HAPG optics on the base of thin flexible films enables:
Fig.4. Thin flexible HOPG/ HAPG films provide unique shape possibilities and can cover a mould of any shape and radii even so fancy as this toy |
- to manufacture crystals of any required shape with radii down to few mm that is hardly possible for any other materials (Fig.4)
- to decrease drastically the energy smearing for such crystals due to a high precision of shape that is determined only by possibilities of the mould machining
- to vary crystal thickness from 500 down to 10 microns
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Especially for high-resolution applications material named HAPG with reduced width of Bragg reflections was developed. HOPG and HAPG exhibit a typical compromise between resolution and reflectivity: flexible HAPG has the twice higher spectral resolution, while flexible HOPG – approximately the twice higher reflectivity.
HOPG/ HAPG optics in von Hamos geometry allows
- to reach energy resolution E/ΔE from several 100 to about 7000 (at 8 keV) subject to set-up geometry and type, geometry, thickness and the order of reflection
- to get brightness 102-103 higher than for flat HOPG crystal and 10-102 higher than for any other crystal in the same geometry
- to use the high order of reflections for increasing the resolution and for shifting the analytical region under investigation to higher energies
- to analyze a wide spectral range simultaneously. The energy band can be further extended by shifting along source-crystal-detector axis
- to detect in real time (e.g. single-shot spectroscopy)
- to develop a compact and relatively cheep device, which could be easy moved and installed at a new place
Fields of application:
- X-ray fluorescence analysis with high detector efficiency and resolution over wide spectral range
- Plasma emission spectroscopy: characterization of laser irradiated targets
- EXAFS
- X-ray Emission Spectroscopy (XES)
- Time-resolved adsorption spectroscopy
- Application with extremely high energy load (FEL) due to outstanding thermoconductivity (1700-2000 W/mK) and radiation stability
Additional information is available in MBI-Projects: 3-04 Transient Structures and Imaging with X-Rays and in the references [10, 23-24, 40-42].
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