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Mirror, mirror
Mirrors are useful not only as household items, but also as scientific tools. Scientists at EMBL Hamburg use specially designed mirrors to reflect and focus X-ray beams onto tiny crystals made of proteins or other biological molecules. By analysing the pattern of X-rays diffracted as they cross the crystals, scientists can determine the molecular structure of the proteins that make up these crystals.
The preparation of an X-ray mirror is a complex process involving a technique called ion beam figuring. In this technique, the mirror is polished to an ideal elliptical shape using a focused beam of plasma (argon ions). This is done with nanometre precision: if you scaled up the mirror to the size of St Peter’s Square in Rome, you would find no deviations from the desired shape bigger than the diameter of a small bacterium.
Each mirror is unique and tailored to its beamline. After each round of polishing, the mirror is shipped to a specialised metrology laboratory, where the shape is measured and compared to the desired surface. Metrologists create a map of differences to be corrected during the next round of polishing with ion beam figuring. The whole process can take more than a year. At each step, even a tiny mistake could spoil the surface, making it necessary to start again from scratch.
Using these mirrors, EMBL’s beamlines can focus X-ray beams much better than before. This allows researchers to use smaller crystals or smaller volumes of precious protein samples, and to obtain higher-quality data. These improvements are exciting news, not only for EMBL scientists but also for hundreds of external users each year, who come to EMBL’s facilities to conduct their experiments.
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