Use of a position sensitive detector for data acquisition of synchrotron X-ray diffraction from adsorbed gas monolayers on graphite

The structure of monolayers of gases adsorbed on graphite has been studied by synchrotron X-ray diffraction [1–3]. Exfoliated Union Carbide ZYX graphite was used as a substrate. A spectrometer using a position sensitive detector and a slit configuration increased the data acquisition efficiency compared to that of the triple axis spectrometer on which the measurement was initiated.

The origin of this comes from the finite coherence length of the graphite flakes, which is [4] 1600 Å. This is much less than the 10 000 Å which can be studied by the triple axis spectometer using Ge(111) both as monochromator and analyser [4]. So the resolution of the triple axis spectrometer is far too good for the object studied.

The reason why this is bothering us is that we are studying monolayers of atoms, thus having very few atoms to scatter from, and even though we are working at the storage ring DORIS some of the experiments are on the limit of the doable with the intensity available. As no monochromator and analyser crystal exists which can fill out the gap between 1600 and 10 000 Å the slits configuration was invented.

Although the introduction of slits limited the part of the synchrotron beam we utilized, this was well compensated for by the use of a position sensitive detector. However, this clearly illustrates the need for focusing elements. In some of the experiments the intensity need will also require wiggler lines.