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Designing an Experiment
X-ray Optics
The x-ray flux onto any specimen depends upon the x-ray source, the energy bandpass of
the monochromator crystals, and the size of the beam relative to that of the sample
(i.e. focusing). The choice of x-ray source, either hard-bend magnet or wiggler, is
discussed in a previous section. This section briefly summarizes the relevant properties
of the available monochromating crystals and focusing elements at CHESS. See the beamline
descriptions for a list of which crystals and focusing elements are available for each
particular beamline.
Monochromator crystals
Because of their crystal perfection and survivability in the harsh white x-ray beam,
silicon and germanium crystals are commonly available and used on all CHESS beamlines.
Several different crystal reflections are used to access different angular acceptances,
energy ranges and bandpass properties. The energy range
for a particular crystal depends upon the angular range and geometry of the monochromator
hardware on a specific station. Except for the stations A1 and F1, all monochromators use
the two-crystal fixed-offset geometry shown in a previous figure. See
the individual station
summaries for crystal selections and energy ranges.
Sagittal focusing crystals
As noted on the station summaries, some beamlines have horizontally focused
x-ray
beams using sagittally-bent silicon monochromator crystals. The silicon bender is able to focus a beam down to 1 mm in horizontal size. Users who wish
to use sagittal focusing should make arrangements well before their arrival.
Total reflection mirrors
Mirrors can be used as low-pass filters, to reduce the flux of higher
harmonics that pass the monochromator crystals, and mirrors can be bent for vertical
focusing of the x-ray beam. At this time, vertical focusing mirrors are only available
for beamlines A1 and F1. Two new mirrors are being constructed for beamline A2: one will
provide cylindrical focusing (vertical only) and the other toroidal mirror will allow
focusing in both horizontal and vertical directions.
The table below lists the mirror types available at CHESS. The measurement at right
demonstrates the use of a float glass mirror as an energy filter.
CHESS Mirrors
The Mirror Constants
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Mirror Type
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Mirror Constant (keV.mrad)
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Float Glass
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30.6
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Silicon
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31.0
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Quartz
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33.2
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Ni-coated
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59.3
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Rh-coated
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67.0
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Au-coated
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80.0
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Pt-coated
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83.0
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The listed energy-critical-angle constant can be used to calculate, for an arbitrary
mirror angle, the energy below which the mirror will have near unit reflectivity.
Last Update:
2006-09-14
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