Beamline Optics


Optical elements
Mirrors VCM/VFM
Mirrors Specifications

Double Crystal Monochromator (DCM)

Optical elements
The optical components of the XAFS/XRF beamline, their functions and there position from the source are listed here.

Components Position (mm) Notes
Diaphragm 4100 Acceptance of 3x0.6 mrad
White Beam Slits 7970 Water cooled slits
Filter 8650 3 C filters with different thicknesses: 50 µm, 200 µm and 1 mm
Be window 11340 water cooled 250 µm thickness Be window
Mirror (VCM) 12678 -Upwards reflecting mirror
-collimating the beam for a better energy resolution
-Double stripes Si and Pt coating
-Dimensions:120x7 cm2
Double Crystal Monochromator (DCM) 15178 -The axe of rotation is the center of the first crystal.
- 1st crystal water cooled
-Dimensions:10x4 cm
-Sagital focusing 2nd crystal
- offset between the 1st and 2nd crystal is 18 mm
 Monochromatic slits  16606  Slits and BPM are mounted on a the same vacuum chamber
Mirror (VFM) 18328 Slits and BPM are mounted on a the same vacuum chamber
Sample position (S1) 30750 -Measure current (detector)
-allow visualization of the beam shape (fluorescence screen “Yag crystal”)

Permit Access to the experimental end station

 The function of the beamline optics is to deliver a synchrotron radiation beam of the desired characteristics to the experimental end-station. In the case the XAFS/XRF beamline the things are quite different because the optics components exist already (donated by ROBL at ESRF). The figure below summarizes the general layout of the beamline optics.

BASEMA-beamline-optics
Top view of the XAFS/XRF beamline optics component showing the Beryllium window (Be), Vertical Collimating Mirror (VCM), Wire Monitor (WM), Double Crystal Monochromator (DCM), Mono Slits (MSLT) Vertical Focusing Mirror (VFM), Beam Position Monitor (BPM) and Photon Shutter (PSh)

Mirrors
Two bendable mirrors M1 and M2 will provide vertical collimation and focusing of the beam with lengths close to 1.2 m located at 12.678 and 18.328 meters from the source point, respectively. These mirrors are made of silicon with a coated layer of about 100 nm of Si and Pt. Figure below presents the Reflectivity versus Energy for Pt and Si coatings of the mirrors at 0.16° (2.8mrad) incident angle. To cover the largest energy range, both mirrors are coated with Si and Pt. In the energy range 4-30 keV, Pt coating present a discontinuity at the Pt L edges (11564, 13734 and 14353 eV for the LIII, LII and LI respectively). Using Si coated part of the mirrors can easily cover these discontinuities.

The mirrors can be remotely adjusted in five independent degrees of freedom. The rotations movements (pitch, roll and yaw) are realized by means of software pseudo motors. The UHV compatible translation stage installed in the vacuum chamber with a UHV compatible stepper motor allows horizontal translation of the mirror. Thus, coating can be changed between Pt and Si. Vertical translations are performed by means of vertical jacks that also perform the pitch and roll rotations.

Mirrors Specifications

Specification VCM (M1) VFM (M2)
Dimensions (mm) 1200 x 150 1200x150
Coatings Si, Pt Si, Pt
Angle of incidence Variable Variable
Substrate Silicon single crystal ZERODUR
Cooling Water cooled Uncooled
Flatness Cylindrical Cylindrical
Type of bender Pneumatic Pneumatic
Minimum radius (km) 5.3 5.3
Surface roughness < 0.5 nm r.m.s. < 0.5 nm r.m.s.
Slope error 1” r.m.s. < 5”r.m.s. (sagittal)
 

Mirror-reflectivity
Mirror reflectivity as a function of the incident angle for Pt (left) and Si (right) coatings

Double Crystal Monochromator (DCM)

The optics of the DCM reflects the beam upwards (positive offset). The Goniometer turns counter-clockwise for positive Bragg angles (looking directly to the optics). Two sets of silicon crystals, Si (111) and Si(311), are used where the crystal cut can be ex-situ changed. The Si(111) allow to access lower enrgies (down to ~4 keV) and the Si(311) allows reaching the higher energies (up to ~30 keV).

The DCM has a dual function:

  • Selecting the appropriate energy and energy band-pass from the incident white (pink) beam
  • Performing a horizontal focusing of the beam thanks to the sagittal focusing second crystal
A third function can be added for higher harmonics rejection by detuning the parallelism of the crystals. This function is subject to more than one constrain. It is applicable only for working at lower energies and only in the case of non-focused beam by using a flat second crystal.
The DCM has two parallel crystals with fixed entrance. The incident beam will come nearly horizontal with a positive angle of about 5.6 mrad due to the grazing angle of the mirror. The beam path in the monochromator is indicated in Figure below.
 
Schematic-beamSchematic beam pass in the double crystal monochromator
 
The outgoing beam is higher than the incoming one by h = 18 mm. The center of the fan of the incoming beam defines the vertical reference plane (Y,Z). The mechanical principle results in one rotation centered on the surface of the first crystal and the rotating plateau bearing two independent translation motion. These move the second crystal in height and beam direction in order to center the beam on the crystal from one hand and to keep the output height of the photon beam constant whatever the transmitted energy.
 

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