Optics - Soft branch

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experiments hutch   safety shutter BPM focusing
monochromator collimating
Lay-out of the soft-line optics


Schematics of the soft-edge branch


The optics for this branch starts with a collimating mirror with Si and Pt strips. This mirror is bent to its maximum bending radius (~20km). The soft branch will allow protein crystallography and XAS experiments to be performed.
The second optical element is a double-crystal (Si 111) monochromator. This is followed by a meridionally focusing mirror with Si and Pt strips. This mirror can also be operated flat in order to achieve the highest angular-resolution. The energy range that can be achieved by this monochromator is 5-40 keV. However, it is possible to reach an energy of 45 keV by reducing the offset (10 mm) between the two crystals.
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).

In order to keep the beam position constant on the sample, a feedback system connected to a beam position monitor is used. This maintains the beam position fixed on the sample during an EXAFS scan by adjusting the gap between the two crystals of the monochromator.

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

Station S
Optical elements Mirror1 Double-crystal mono Mirror2
Distance from source 26.0 m 28.6 m 31.5 m
Type vertical collimator
pure Pt/Si strip coating
sagittal focusing of the second crystal meridional focusing Pt/Si coating
Energy range 5-40 keV    
beamsize at sample 0.5 mm x 0.5 mm2 (V x H) [calculated]    
Flux 1 x 1011 photons/sec    
Energy resolution dE/E 2 x 10-4    
Horizontal acceptance 2 mrad    


See also:

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