Monochromator

Multilayer [Ru/B₄C]₁₂₀ was deposited inhouse on two Si-substrates 300mm long with period of 2.96 nm at centre. The monochromator is a ‘nearly’ fixed-exit (beam moves by only 5 microns when changing energy from 7 to 15 keV) UHV-compatible device, scattering in the horizontal plane with a distance of 9 mm between the monochromatic and the incident white beams. Measured resolution deltaE/E at 8 keV is 1.6%. The photograph on the left shows the monochromator vacuum vessel as installed in the Optics hutch. The image on the right shows intrenal parts of DML with water cooling tubes.

Energy mainly used is 12.5keV and can be easily modified by typing desired number (between 7 and 15) in dedicated field in BsxCuBE GUI. Whole system is than rotated by a tiny angle to do so. No additional "rocking curve'' tuning scan of second multilayer is needed (very large Darwin width) and not recommended as 2 MLs don't have exactly the same spacing (1st ML ~ 2.96nm and 2nd ML ~ 2.92 nm).  A small rotation of 2nd ML will produce quite big horizontal beam shifts (several mm to cm) downstream from monochromator. The beamline was aligned with respect to monochromatic beam obtained when 2nd ML rotation corresponds to maximum intensity at 12.5 keV.

A watercooled beamstop is integrated into the monochromator vessel and serves to block any of the white beam escaping from the first ML but allows the monochromatic beam to pass. The size of the X-ray beam at the entrance of the monochromator is typically 4 mm  4 mm.

Morors linked to the DML monochromator:

• ml_y: translation perpendicular to the X-ray beam, locked (and normally not visible in spec) via two brakes, how to unlocked it and move see BM29 wiki (only if 120% sure what you are doing ;-)
• ml_rot: rotation of whole vessel, setting energy, it is very small rotation (angle between first ML and beam being 0.8º at 15 keV and 1.714º at 7 keV).
• ml_push: small rotation of 2nd ML

Energy calibration

The energy calibration system installed just behind the monochromator can introduce by rotation into the beam metal foils of Fe, Cu and Pt, which have been chosen as they have absorption edges inside the accessible energy range (7.112, 8.98 and 11.564 keV, respectively). A retractable diode inside the rotating filters device allows to measure these absorption edges while rotation monochromator.

Calibrator motor is 'cal' and each unit means a different position, 0 (and 4) being without metal foils, other position are filled by:

• position 1 and 5: Fe
• position 2 and 6: Cu
• position 3 and 7: Pt

Ex: OH> mv cal 3 will put Pt-foil in the beam

Diode in calibrator's centre part is set in/out of beam by 'calibin/calibout'.