Procurement of synchrotron microtomography scanning of fossils

contract award notice

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Uppsala University intends to sign an agreement with European Synchrotron Radiation Facility in Grenoble for performing synchrotron microtomography scanning of fossils.   Background and motivation Synchrotron microtomography scanning of fossils is used to visualise the internal three-dimensional structure of the fossils, allowing these to be studied without damage. The technique is central to the research of the Programme of Evolution and Development and underlies a large proportion of their publications.   Imaging needs to be done at a range of resolutions from approximately 40 µm (for morphology large objects) down to 0.7 µm (for detailed examination of histology), with an option to carry out multi-scale scanning on one specimen without dismounting it from the apparatus. In order for the research to be completed successfully it is essential to obtain the highest possible image quality, with sharp outlines, no beam-hardening artefacts (which artificially brighten the edges of the scanned object), and maximum contrast between bone and surrounding rock. This can only be achieved by propagation phase contrast synchrotron microtomography; conventional microtomography using non-synchrotron x-ray sources does not produce adequate results. The differences can be summarised as follows:   ·          The difference between synchrotron radiation and conventio-nal x-rays is similar to that between a laser and an ordinary light source such as a torch: synchrotron x-rays are monochromatic (i.e. just one wave length and energy), coherent (the waves all move together in phase), and perfectly parallel, whereas a conventional x-ray source produces many wavelengths and energies, which are not coherent, and the paths of the photons diverge in all directions. The result is that conventional x-ray scans produce what are known as "beam-hardening effects", whereby low-energy photons are absorbed selectively. The visual conse-quence is bright foggy edges to the blocks containing the specimens, which obscure the contents within. Synchrotron scans are much sharper and lack beam-hardening effects.   ·          Because synchrotron x-rays are monochromatic and coherent, they can be used for detection of propagation phase contrast. This means detecting not just the absorption of x-rays by the specimen (which is all you can do with a conventional x-ray source), but also the refraction effects created by the structures inside the specimen. The refraction causes photons to diverge from their original path, an effect that becomes more pronounced the further the photons travel after passing through the specimen before hitting the detector; this is the "propagation distance". Because a synchrotron scan detects both absorption and refraction, its ability to distinguish different materials (such as bone embedded in rock) is several thousand times greater than that of a conventional x-ray scan with the same resolution. It is commonplace for a synchrotron scan to show a wealth of internal anatomical structure where a conventional x-ray scan revealed nothing at all.   Synchrotron microtomography is thus essential. In addition, the synchrotron beam needs to be powerful enough to penetrate rock specimens of at least 20-30 cm diameter, which requires a beam with a minimum energy of at least 200 KeV (kilo electron volts), and at the same time wide enough to be usable on specimens of that size. This is limiting, because mot synchrotrons produce beams of smaller size and/or lower energy. For example, MAX IV reaches a maximum energy of no more than 30 KeV and is thus no use to us.   Alternatives There are many synchrotrons in the world, but only a few offer tomography and only two, ESRF (European Synchrotron Radiation Facility, in Grenoble) and SLS (Swiss Light Source, located between Zürich and Basel), have specialised tomographic beamlines suitable for palaeontology. The TOMCAT beamline at SLS is excellent for small specimens but cannot manage objects more than about one-tenth the size we require. By contrast, ESRF Beamline BM18 has been built specifically for performing multi-scale propagation phase contrast microtomography on large specimens, and has by far the largest specimen stage, widest beam, and longest propagation distance in the world. Beamline BM05, operating in tandem with BM18 under the same management team, is specialised for very high resolution. No other tomographic facility comes close to matching these capabilities.     Decision On the circumstances above, it is decided to procure synchrotron microtomography scanning of fossils from ESRF in Grenoble, by negotiated procedure without prior advertising on the basis of technical exclusivity. The contract value is estimated to be 2 MSek.

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