The radiation fan is split into two branches which each accept 2 mrad, and can be operated simultaneously. One branch (on the hard-edge of the bending magnet) is devoted to Small and Wide-Angle X-ray Scattering (SAXS/WAXS) and Interface Diffraction (ID), while the other (on the soft-edge) is equipped with XAS and Protein Crystallography equipment.

Schematic layout of the optical components of BM26

Optics - Hard branch

Lay-out of the hard-line optics

Schematics of the hard-edge branch (Click on the picture for details)

The two techniques on the hard-edge branch (SAXS/WAXS and ID) use focused, monochromatic radiation with a moderate energy resolution.
The first optical element is a double-crystal monochromator with sagittal focusing on the second crystal it is followed by a meridionally focusing mirror with an angle of incidence of 2.8 mrad. Both focusing units are adjustable, so that the X-ray beam can be focused from the SAXS/WAXS sample position (48.6 m from the source) to the centre of the ID diffractometer (57.6 m from the source).

Two different mirror coatings are used to provide the discrimination against higher harmonics without changing the mirror angles. Using the Pt mirror coating, the beamline operates from 12-30 keV, while with the Si coating the operational energy is 5-12 keV. Together with the possibility of detuning the monochromator, this should yield a harmonic content of less than 10 ^-4 using Si(111) or Si(311), except for very low energies. The appropriate mirror coating is positioned in the beam by a sideways translation.

The characteristics of the optics of the H station are summarized here.

The experiments hutch contains the SAXS camera with a maximal and minimal camera length of 8 and 1.4 m, respectively. In parallel a WAXS detector based on a curved microstrip glass strip counter is installed. This allows simultaneous SAXS/WAXS measurements with a time resolution down to 1 msec/timeframe. Furthermore a heavy-duty '2+3' type diffractometer (combined '2+2' and 'z-axis') for interface and surface diffraction experiment allowing horizontal and vertical sample orientation is mounted in the same hutch.

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Optics - Soft branch

Lay-out of the soft-line optics

Schematics of the soft-edge branch (Click the picture for details)

The optics for this branch starts with a collimating mirror. The soft branch will allow protein crystallogrphy and XAS experiments to be performed.
The second optical element is a double-crystal monochromator. This is followed by a meridionally focusing mirror. The meridionally focusing mirror can also be operated flat in order to achieve the highest angular-resolution.
For XAS experiments the monochromator is continuously scanning, making it more difficult to guarantee a constant harmonic rejection ratio. For this reason two mirrors are used in the soft branch for harmonic rejection. Similar as for the hard-edge, the two coatings are Pt and Si. For XAS it is important to have an incoming beam without any absorption edges, and the L-edges of Pt are a potential problem. However, by choosing the incoming angle for both mirrors to be 2 mrad, the Si coating will yield a 'clean' beam in 5-15 keV energy range, i.e. beyond these L-edges. The Pt coating will cover the spectral range of 15-30 keV. The harmonic rejection from the two mirrors is about a factor of 10 better than that of the single mirror in the hard branch. Assuming a less efficient detuning of the monochromator, the total harmonic rejection should still be around 10 ^-4, i.e. similar to that of the hard branch (taking monochromator crystals with only third and higher harmonics).

The characteristics of the optics of the S station are summarized here.

See also:

Center for X-Ray Optics at LBNL
Calculate scattering length densities

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