July 2005

X18B: A New Monochromator for X-Ray Absorption Spectroscopy Between 4.9 and 40keV

A new monochromator has been installed at beamline X18B to lower its lower energy limit from 5.6keV down to 4.9keV. This change might sound small, but it opens significant opportunities for research - especially in solid-state physics, materials science, and catalysis research - since the K-edges of two important 3d-transition metals, titanium and vanadium, have their energies at 4.966 and 5.465keV, respectively.

New X18B Monochromator with smaller first crystal.

Beamline X18B is optimized for hard x-ray absorption spectroscopy. The original monochromator covered an energy range between 5.7 and 40keV (Cr - Ce K-edges, Ce - U L-edges). An overlapping lower energy range (2-7keV) is covered by beamline X19A, which is optimized for x-ray absorption spectroscopy below 4keV. However, several research groups, especially in the field of catalysis research, do experiments at the K-edges of several 3d-transition metals. The typical duration for one of their experiments is two to three days, including a few hours of research at photon energies below 5.7keV (i.e. the V and Ti K-edges).

Rather than attempt to schedule and setup experiments at both X18B and X19A, we decided to extend the lower energy range of X18B down to 4.9keV via a simple modification of its monochromator. The monochromator at X18B is a standard channel-cut monochromator with a 3mm gap between the two crystals. The original length of the first crystal was 34mm, which limited the maximum usable angle of operation of the monochromator, and in turn limited the low end of the photon energy range to 5.7keV. The 34mm length of the first crystal was chosen in order to intercept the entire beam vertically at higher energies, where the Bragg angle, and thus the vertical acceptance of the crystal, is smaller.

However, as it turns out, the vertical opening angle worsens the energy resolution at higher energies, so it is better to limit the vertical beam size at high energies, thereby shortening the required length of the first crystal and increasing the usable angular and photon energy ranges of the monochromator. By optimizing both the energy resolution and intensity, the footprint of the white beam on the monochromator remains almost constant over the energy range of this monochromator. This allowed us to shrink the length of the first crystal to 16mm, which is still enough to prevent heat-load problems like the thermal bump.

The monochromator was first mounted in the beamline in the end of April, and tested and commissioned in April and May.