G2 Beamline

High resolution, thin-film x-ray diffraction

G2 Station Summary
First Monochromator: Two-bounce, 1.7% multilayers
Second Monochromator: Side-bounce, transmission Laue Berylllium (220)
Energy Range: 8-13.6 keV
Typical beam size: 2.0 mm HZ by 0.2 mm VT
Typical flux: 1011 photons/second
Bandwidth: 0.1 %
Typical Applications: High resolution Grazing Incidence Diffraction, X-ray reflectivity, off-specular CTR

G2 is one of five CHESS hutches fed by a single 49-pole wiggler in the Southwest section of the CESR tunnel. G1, G2, and G3 are fed by the positron beam, whereas A1 and A2 are fed by electrons. The front-end G-line optics consist of an interally water-cooled collimating mirror, two pairs of synthetic multilayer monochromators, and two additional, monochromatic mirrors. The wiggler beam is separated by the upstream multilayers of each pair into upper and lower branches. The upper branch is delivered to G1, whereas the lower branch is used by both G2 and G3. The collimating mirror and monochromatic mirrors are both mounted to dynamic benders, providing vertical focusing. Horizontal, sagittal focusing is provided by the cylindrically-shaped second multilayer. The G2 beam is split from the G3 beam by a secondary, side-bounce monochromator operating in transmission geometry, comprised of a thin section of a Beryllium single crystal. The geometry of the G2 monochromator generally results in a horizontally divering beam, but the vertical focusing is retained. As a result, G2 delivers a beam that is naturally wide horizontally but vertically narrow.

The natural G2 beam shape is well suited to working at grazing incidence, and therefore for the study thin films. The G2 beam characteristics are complemented by custom, in-hutch instrumentation, especially a Kappa-style, six-circle diffractometer. This diffractometer is ideally suited for thin film work, since it allows the sample accessing a large range of reciprocal space while keeping both the incident beam angle and foot-print on the sample constant. Compared to many GID setups, which utilize 2D detectors, G2 combines a set of soller slits with a tall 1D diode array, which eliminates degradation of in-plane scattering angle resolution due from the large beam foot-print arising from grazing incidence geometry (Figure 2).

G2’s dinstinctive characteristic is the combination of a setup optimized for thin film characterization, with its ease of use. A vacuum sample chuck and wall-mounted alignment laser allow fast sample exchange. A helium shroud is available both to reduce background and for samples, such as very thin organic films, that are attacked by ozone.

Figure 1: G2 Kappa diffractometer, illustrating sample goniometer, soller slits and 1D diode array.