The Nanoscopium beamline, a technological challenge

Location of the future Nanoscopium beamline

The energy range covered by Nanoscopium will be 5-20 keV (hard X-rays), allowing access by absorption spectroscopy to elements from titanium (Z = 22) to molybdenum (Z = 42) for the K-edge, and iodine (Z = 53) to uranium (Z = 92) for the L-edge. In addition, X-ray fluorescence spectroscopy will give information on elements with atomic numbers as low as phosphorous (Z = 16).

Techniques that will be available on Nanoscopium require the use of coherent X-rays, both to obtain a tightly focused beam of only a few tens of nanometers in diameter, and to enable the implementation of the analytical techniques chosen for the beamline. The "primary" X-ray beam produced by the undulator is first rendered monochromatic, and then passed through a slit, called a “secondary source”, which adapts the beam characteristics (coherence level, size and intensity) to the needs of the focusing optics and the experimental techniques. The distances between the primary and secondary sources, and also between the secondary source and the sample, are very large – around 80, and 60-70 meters, respectively. A traditional beamline measures about 40 meters, but Nanoscopium is much longer in order to obtain a more coherent and very narrow beam in the experiment stations.

The extreme mechanical and thermal stability of beamline equipment, and the buildings themselves, is paramount, given their length. The beam travels a distance of 160 meters but its size and position on the final sample must be accurate to within tens of nanometers, so interference from any ambient vibrations or thermal drifts is out of the question! This explains the special care given, on the one hand, to the construction of the optics and experimental stations. The foundations were designed following highly detailed and sensitive vibration studies of the ground, and the hutches will rest on thick concrete slabs themselves raised on piles driven down 16 m - like the rest of the synchrotron and beamlines. On the other hand, an air conditioning system common to all hutches, plus local thermo-regulated enclosures, will ensure temperatures stable to 0.1° C for the optics and experimental stations. 

The first Nanoscopium experimental station, run by Cameron Kewish, will be optimized for coherent diffractive imaging. A submicron beam (300-400 nm) will be used to probe the sample to obtain its structure with a final resolution below 30 nm. Cameron will bring his expertise in 3D imaging using coherent diffraction to the beamline (ref Dierolf et al., Nature 2010, Vol 467, 436-439) developed particularly during his collaboration with groups in Munich and on the cSAXS beamline at SLS, which is optimized for coherent X-ray scattering techniques.

Scanning coherent diffraction imaging or "ptychography" is a very new technique (ref Thibault et al., Science 2008, Vol 321, 379-382), which provides information on the structure of the sample from a series of diffraction images obtained with a fully coherent beam, followed by the use of specific algorithms. This technique also provides 3D images: the sample is rotated in the beam during the experiment and is analyzed from different directions - as is the case for tomography. In the case of full-field tomography, each view through the sample is the equivalent of a medical radiograph - a picture showing the difference in absorption or refraction of the X-rays by different parts of the sample. Ptychography, meanwhile, offers another approach. The 3D reconstruction is based on the "compilation" of hundreds of thousands of diffraction images collected as the sample is scanned through the beam: for each orientation, hundreds of patterns are recorded, in order to reconstruct a 2D image of an X-ray wave passing through the sample. All the 2D images are then used for the final 3D reconstruction. This procedure requires the implementation of several types of reconstruction and visualization algorithms on particularly powerful computers! The next step, concerning data processing, is to treat directly and simultaneously the ensemble of diffraction patterns, without first obtaining the 2D reconstructions of the images. Such algorithms are still under development.

The end result gives not only high resolution, but also quantitative information on the sample because the data collected are directly related to the electron density distribution in the sample. This quantitative aspect is crucial, for example, in the context of osteoporosis research (bone density).

This technique is of interest, among others, to more and more users in the medical field, who will find, on Nanoscopium, the most advanced equipment in this field. A 2D pixel detector (e.g. XPAD or PILATUS ), specially adapted for the energy range of the beamline, is planned and the station will have its own dedicated computer servers so that the impressive amounts of data collected during each experiment can be stored. The simultaneous use of scanning fluorescence spectroscopy and X-ray absorption imaging is also planned for this station, with a spatial resolution determined by the beam size (i.e. 300-400 nm).

Andrea Somogyi, Head of the Nanoscopium beamline, will be responsible more specifically for the second station, dedicated to the coupling of scanning X-ray imaging techniques using an X-ray beam focused to a size of 30-100 nm. It will be possible to simultaneously obtain high resolution and high sensitivity elemental distribution maps (detecting trace elements) by means of this real nanoprobe. This station will offer users the possibility of having, all on the same sample under the same experimental conditions, structural information by the differential phase contrast method, the distribution of elements by X-ray fluorescence and chemical information from X-ray absorption spectroscopy.

The data will be recorded by a suite of detectors specific to each technique: X-ray fluorescence detector array for fluorescence experiments and absorption spectroscopy, the pixel detector and / or segmented diode for differential phase contrast measurements. The experimental station will also be equipped with a cryofreezing system suitable for analyzing biological samples at temperatures down to 110 K, to minimize structural changes to the sample induced by the high intensity X-ray beam.

Biology, biomedicine, geobiology and the environmental sciences will be the main areas of research on this beamline, allowing, for example, metals to be localized in tissues or cells, to image and analyze sub-cellular compartments or microorganisms, including within mineral samples.

Nanoscopium represents a major technological challenge: very strong constraints are imposed in terms of mechanical and thermal stability, optics, detection and data processing. The features, specifications, and new X-ray imaging techniques expected for Nanoscopium have motivated all groups involved in the design and construction of the beamline to work to make the line available to users by early 2013.