[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The measured angular diffraction spectra in

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The measured angular diffraction spectra in comparison with ray-tracing calculations (left) and a reconstructed flux distribution in a focal plane (right) In Fig. 3.65 (right) is shown a reconstructed flux distribution in a focal plane obtained by Fourier transformation of the calculated ray-tracing spectra and the shape of the experimental spectra. Both curves have the same Full Width of Half Maximum (FWHM) of 0.3 µ m. Analysis of an angular distribution of diffracted rays leads to the possibility to fit experimental spectra and find zone plate characteristics, such as outer zone width and thickness of a phase-shift layer. These characteristics can be obtained almost independently from the experimental conditions, taking into account only beam divergence. Using this method considerable resolution improvement has been demonstrated for the meridional Bragg–Fresnel lens in comparison with a normal-incidence zone plate with the same outer zone width. The resolution enhancement of 4.3 times was measured experimentally. The measured efficiency of the Bragg–Fresnel zone plate with a thickness of 190 nm corresponds to the efficiency of a transmission zone plate with a thickness of 1,350 nm. Bragg–Fresnel Optics Applications At the new-designed BESSY Microfocus Beamline, the horizontal beam focus- ing is achieved with the help of a sagittal Bragg–Fresnel lens for micro small angle X-ray scattering ( µ SAXS) experiments (Fig. 3.66). Several Bragg–Fresnel lenses are placed on the second monochromator crystal, in our case Ge (111), which provides about 30% higher flux in com- parison with a Si (111) crystal. A linear Bragg–Fresnel lens on a crystal substrate was suggested and tested several years ago [183]. The first tests were done on the structures etched in a Si crystal to produce a π phase shift, necessary for an effective diffraction focusing. The main problem in using such a structure for microfocusing was a very large depth of profile necessary to achieve the optimal phase shift. For Ge (111) at 10 keV the depth reaches 0 . 6 µ m. One needs to use an etching
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X-Ray Optics 185 First crystal (Ge) Second crystal (Ge) with a Bragg Fresnel lens ~3500 mm Bimorph mirror Fig. 3.66. Small Angle X-Ray Scattering focusing system employing a Bragg– Fresnel lens and bimorph mirror at BESSY technology with a very high aspect ratio to obtain a reasonable lens aper- ture, which is defined by the minimum zone width. Several attempts were made to produce a special “incline” profile to improve zone plate acceptance and resolution. Recently this problem has been solved using metal coating technology. A Fresnel structure was fabricated using a metal coating on a perfect Si (111) crystal in the shape of a Fresnel zone plate. In the case of such a coated zone plate the beam is transmitted twice through the thickness of a metal coating at the Bragg grazing angle θ B . In comparison with transmission zone plates the value of the optimal thickness is reduced by the factor of 0 . 5 sin( θ B ).
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  • Spring '14
  • MichaelDudley

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