Beamline 12.2.2 uses a APM Kohzu monochromator with custom in-house additions for the crystals and multilayer optics.
Schematic layout of the monochromator elements within the Kohzu
monochromator.
The basic monochromator rotates all optics about the central axis that runs along the surface of the first crystal. This rotation is the main θ drive that determines the monochromator angle and thus X-ray energy selected. The first crystal is fixed to this main rotation platform, whilst the second crystal is mounted off a stack of four stages that in turn is supported from the main θ rotation platform. The four stages that the second crystal rides on can be adjusted to maintain the constant beam height offset required for the beamline optics.
The first multilayer is fixed to the first crystal support structure and, as shown in Figure above, is mounted upstream of the first crystal. The second multilayer is mounted on the stack of four stages along with the second Si(111) crystal. By rotating the main θ drive to the low grazing angle required for the multilayer, the off-axis location allows it to intercept the beam before the Si(111) crystal and directs the X-rays to the second multilayer (see Figure above). Of course the off-axis location of the first multilayer means that the X-rays will walk off the surface, but by suitable choice of mirror length (176 mm) and d-spacing (2.0 nm) it is possible to achieve a useful energy range of ~14–25 keV before the beam has walked too far off the mirrors. The multilayers used were 150 layer pairs of W/B4C supplied by Osmic with figure error <1 mrad r.m.s.
- Si(111): The silicon crystal offers an energy range between ~ 6 keV and ~ 40 keV. It's resolution E/ΔE is ~ 7000. It has a maximal flux of about 1 x 1011 photons/sec around 25 keV.
- Multilayer: The W/B4C multilayer monochromator covers energies between 14 and 25 keV. It's energy resolution is ~ 120, which is accompanied by a proportional increase in flux compared to the Si(111)
