Description of the D2AM optics

The optic has a symmetrical design and consists of:

  • Two 1.1 m long mirrors coated with two strips of 400 Å-thick and 30 mm-wide Rh and Pt along the whole length.
    • An initial water-cooled mounted in the horizontal plane 26 m away from the source, allows a vertical reflection of the incident polychromatic beam. The incidence angle is set slightly below the critical angle of total reflection for a wavelength λ to reject its λ/3 harmonics. The mirror can be cylindrically bent in order to parallelize the reflected beam.
    • A second mirror reflects the monochromatic beam downwards. This reflection, coupled with a rotation of the mirror around a horizontal axis perpendicular to the beam direction, puts the beam back on its initial horizontal axis defined by the source. An ellipsoidal bending allows focussing the beam vertically. Without using any other focusing lenses or mirrors.


  • A two-crystal monochromator (Si(111) or Si(311)) located between the two bent mirrors in a compact arrangement, with double-focusing (3:1):
    • An initial Si (111) water-cooled crystal mounted 1.93 m downstream of the first mirror
    • A second Si (111) crystal, mounted on a specific sophisticated mechanical system, leading to an energy resolution Δλ/λ = 2 × 10−4. This tower is designed to allow not only tilt and position tuning of the second crystal but also it's bending in order to focus the beam sagittally. This crystal was locally backside-etched to shape segments of 700 µm-thick Si, separated by non-etched ribs with 400 µm widths and 3 mm thicknesses. A rib-free central zone 5 mm in width is mainly used to deliver a uniform, focused beam without the satellites that may be reflected by the inter-rib regions. The tuning of the second crystal is permanently monitored and adjusted using a feedback tracking system where a piezo motor adjusts the Bragg angle of the crystal to maintain the highest diffracted intensity when scanning the energy.

Having the rotation axis of the monochromator system set on the surface of the first crystal, together with the available translations of the second crystal, delivers a point-focused fixed exit beam essential for energy scans. It is, therefore, possible to perform anomalous X-ray scattering by maintaining the illumination on the same spot on the sample.

Main specs with this configuration:

  1. A flux of 1011 photons/s in a 100 µm × 90 µm (horizontal × vertical) focal spot size.
  2. Point focusing of the beam on the crystal at a fixed sample position.
  3. High energy resolution: 2-5 eV; E/E~ 2.10-4
  4. High Q resolution; DQ/Q~ 10-3 - 10-4
  5. Accessible energies 5-45 keV;
  6. A high rate of harmonic rejection: higher than 10-4.
  7. With focusing typical angular divergences are 3.10-4 x 3.10-3 rd2 (vertical x horizontal) for a beam size of 3*30 mm2 on the monochromator.

Distances between optical elements:

  • source - primary mirror: 25.935 m
  • source - monochromator : 27.867 m
  • source - second mirror: 29.686 m
  • source - goniometer center: 39.261 m
  • source - beamline slits: S1 --> 24.215 m ; S2 --> 31.436 m ; S4 --> 38.441
  • source - SAXS slits: 40.819 m