Topography - Microtomography beamline
X-ray imaging started over a century ago, at the same time as the discovery of X-rays. For several decades its only form was radiography. Other imaging techniques were developed in the last decades: X-ray diffraction topography to characterize crystals for the microelectronics industry, medical scanners for a "three dimensional" view of the inner part of human body. The modern synchrotron radiation sources completely renewed X-ray imaging. They allowed exploiting new possibilities (very high spatial, angular or time resolution imaging), and also led to the emergence of original techniques, like "phase contrast" imaging associated with the "coherence" of the beam. ID19 is a multi-purpose long (145 m) imaging beamline for radiography (absorption and phase contrast imaging) microtomography, and diffraction imaging (topography, analyser-based imaging) experiments.
The beamline is installed on a low-beta section of the storage ring. This allows having a small source size (30 µm vertical x 120 µm horizontal). Three insertion devices are located in the ID19 straight section (two undulators and one wiggler) the choice of one of them as source being a function of the experiment requirements. Long beamline The length of the beamline coupled with the small source size allows 1) exploiting the coherence properties of the beam, and 2) having either a wide and homogeneous beam when needed, or a focus spot below 100 nm when required
Energy range and monochromators
• The beamline can work in the energy range 6 to 100 keV, but most of the experiments are performed in the 10-35 keV range
• The monochromatic beam can either originate from a double Si 111 crystals monochromator (ΔE/E 10-4) or a state of the art multilayer (ΔE/E 10-2)
Various setups designed for the different imaging techniques are permanently installed in the experimental hutch of ID19. They are
• A microtomograph designed to perform absorption microtomography and, by varying the sample-to-detector distance, phase microtomography
• A « horizontal » diffractometer, designed for experiments requiring white beam and/or bulky sample environment (furnaces, cryostats, magnets, …)
• A « vertical » diffractometer that includes the possibility of using and analyser crystal, for very accurate monochromatic beam imaging and diffraction
Scientific and Industrial Applications
The use of the properties of the beams delivered by third generation SR sources, and more in particular the ESRF, is pushing up dramatically, and in many respects well beyond expectations, the possibilities and applications of 2D and 3D X-ray imaging. These techniques are applied both to scientific problems (in physics, materials science, environment, biology, medicine) and to purely industrial ones (cosmetics, foams, reservoir rocks, composite materials, …). The applications are based, as far as absorption imaging is concerned, on the very broad choice available in the photon energy (typically between 6 and 120 keV), which makes it possible to improve the contrast, on the improved spatial resolution (on the order of the µm), and on the quantitative data evaluation allowed by the monochromatic and parallel character of the beam. The very small source size provides, in an instrumentally simple way, phase images that reveal phenomena that are difficult to evidence by other means. This technique can be used for 2D or 3D imaging, either qualitatively for singularity edge detection (pores, inclusions, ...), or quantitatively, through phase reconstruction complemented by tomographic reconstruction, in the approach called holotomography. The small source size coupled with the improved focusing devices lead to the development of scanning imaging techniques, which allow the detection of trace elements. Diffraction imaging (X-ray topography) remains a unique tool to characterize single crystals and to investigate many crystal physics phenomena (creation of defects, domains, phase transitions, vibrations, …). Its scope is extended to unexpected topics by the use of coherent beams. These techniques, and their combinations, are well adapted to in-situ experiments, where the material, in an adequate sample environment device, is imaged while an external parameter (temperature, stress, …) is changed. Some of the applications of course require even better spatial resolution. X-ray detection is performed through films or visible light scintillators. The system is then diffraction limited. A possibility to overcome this limitation is to use an X ray lens to magnify the image before the detector. Promising attempts have been performed using Kirkpatrick-Baez focusing devices.
Phase contrast imaging
The X-ray beams produced at third generation synchrotron radiation facilities exhibit a high degree of coherence. This results from the small source size σ (in the 50 µm range) and the large source to sample distance L (in the 100 m range). The transverse coherence length dc= λL/2σ, is in the 100 µm range, and allows to record “phase images” by just varying the sample-to-detector distance (“propagation technique”). The great advantage of this new type of imaging is the increased sensitivity it provides, either for light materials such as polymers, or for composites made up of materials with neighbouring densities (for example Al and Si). A first use of the phase images rely on the visualisation of the phase jumps that occur at the edges of a particle or porosity imbedded in a matrix having a different index of refraction. A more sophisticated approach (“phase retrieval”) allows obtaining the local phase shift, which is proportional to the density
The principle of microtomography is very similar to the one of the well-known medical scanner. When applied to materials investigation, it consists in recording a series of radiographs (typically of the order of 1000) for different angular positions of the sample, which rotates around an axis perpendicular to the beam. Several laboratory microtomographs have been commercially produced over the last years. But the best images, in terms of spatial resolution, signal-to-noise ratio and quantitative exploitation, are obtained using synchrotron radiation. They result from the high intensity, practically parallel and monochromatic incoming beam. In this approach: there is no image magnification, and the spatial resolution mainly results from the effective pixel size of the detector. The range of pixel sizes available at the ESRF goes from 0.3 µm to 30 µm, and a big effort is being produced to enhance the spatial resolution to the 100 nm range. The total acquisition time is in the few seconds (“fast tomography”) to 1 hour range, and the recorded data is often several Gigabytes. Microtomography can be combined with phase contrast imaging, either in a qualitative way (“edge enhancement”) or, more quantitatively, including phase retrieval (“holotomography”).
X-ray diffraction topography is the generic name for techniques using X-ray beams that have been Bragg-diffracted by a crystal to image it. The resulting pictures are called topographs. They show the distribution, in direct space as in all microscopy techniques, of various singularities that affect the Bragg reflection used, and in particular crystal defects such as precipitates, individual dislocations, stacking faults, domain or phase boundaries. Topography rests on the fact that singularities or inhomogeneities can affect the spatial distribution of diffracted intensity and hence result in contrast. In its usual meaning, topography can only be performed on single crystals, or on single grains within a polycrystal. There is a wide range of variants: in transmission or in reflection, with a monochromatic beam or with a white beam, with a divergent beam or with an almost plane wave. Diffraction topography is basically similar to dark-field electron microscopy. It is very different from electron microscopy in all relevant orders of magnitude: its resolution is considerably less good (in the µm range), but it can handle in a completely non-destructive way samples many orders of magnitude thicker (in the 500µm range). The specific contribution of SR is the enhanced spatial resolution (“weak beam” technique, for instance) and/or the real time experiments (crystal growth, movement of defects, domains or phases, etc…)