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1 4 7 I H I G H L I G H T S 2 0 2 3

New beamline BM18

The new flagship beamline BM18, inaugurated in October 2022, is optimised for phase-contrast imaging of large objects. About 20 years ago, propagation phase- contrast techniques revolutionised X-ray imaging of fossils, being sensitive to much finer density variations. As the density of a fossil can be very similar to those of the surrounding sediment, this extra sensitivity made it possible to show features that were normally invisible with classic X-ray absorption-based techniques. This made it possible to image insects fossilised in amber, fossilised embryonic bones preserved in egg, or the daily increment in hominid teeth, to name a few examples indeed, page 154 gives one such example of high-resolution X-ray microtomography being used to image the gut contents of a fossilised trilobite, allowing researchers to identify its diet.

The idea in developing BM18 was to expand the size of fossils that could be imaged while conserving key aspects, notably a high level of spatial coherence and the possibility to use high energies. Based on a three- pole wiggler, the beam reaches a width of 30 cm in the experimental hutch, located 175 m from the source. This long distance from the source, coupled with the smallest spot size in the current storage ring lattice, makes it possible to use propagation phase-contrast imaging at very high energy and relatively coarse resolutions. Using filtered white beam, it is possible to work with energies ranging from 70 to 260 keV. To maximise efficiency, a series of nine detectors are permanently mounted, offering an almost continuous spectrum of pixel size ranging from 0.7 µm to 67 µm. Finally, the 45-m-long experimental hutch allows for very long propagation distances, necessary for phase-contrast imaging in that range of energies and resolution.

With all these characteristics, it is possible to perform phase-contrast imaging on fossils up to 30 cm in diameter. The set of detectors means the resolution can be changed efficently without having to move the specimen. Tools have been developed to streamline the process of scanning a fossil completely, and to use the low-resolution data to target specific regions of interest at higher resolution. Complete beamline configuration snap-shots can be recorded and recalled by selecting them in a menu, allowing a complete change of configuration (i.e., energy, propagation distance, pixel size) in about five minutes. The properties of the beamlines have naturally attracted palaeontologists, who are offered more possibility and more versatility than ever before.

M. COTTE & V. FERNANDEZ