PHOTON SCIENCE

A secondary double-crystal large offset monochromator (LOM) is installed further downstream of the liquid nitrogen cooled DCM. The LOM fulfills several tasks at the beamline: (i) it lifts the beam up by 1250 mm to separate it from the P09 beam, (ii) increase the resolving power of the X-ray beam, and (iii) suppress higher harmonics by 8 orders of magnitude. In the original design of the beamline the LOM was equipped
with a Si311 and Si511 crystal pair. As an upgrade the size of the Si crystals was reduced and the crystal cage was
modified to install two additional pairs of Ge crystals (311 and 511) in 2015.

Due to the larger bandwidth of the germanium crystals the flux throughput of the LOM is increased by a factor two for photon energies smaller or significantly larger than the germanium K-edge at 11.1 keV. On the other hand the resolution is decreasing and the ability to suppress higher harmonic contamination is reduced to a factor of 10⁶ as compared to 10⁸ in case of using Si.

• For the photon energy range from 5.36keV to 18.8keV the 311 reflection can be used. The incident angle is never smaller than 5.8deg. The loss in intensity at 10keV compared to a silicon 111 monochromator is about a factor of four. On the other hand the energy resolution is increased by approximately a factor of four.


• For the photon energy range from 8.4keV to 29.4keV the 511 reflection can be used. The incident angle is never smaller than 7.96deg. The loss in intensity at 20keV compared to a silicon 111 monochromator is about a factor of ten. On the other hand the energy resolution is increased by approximately the same factor.