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 laminography experiments.
Diffraction imaging (topography, analyser-based imaging) instruments moved in 2007 to BM05.
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). Five insertion devices are located in the ID19 straight section (four undulators and one wiggler) the choice of one of them as source being a function of the experiment requirements. 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 250 keV, but most of the experiments are performed in the 19-35 keV range
• The monochromatic beam will either originate from a double Laue or Bragg 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
• 3 microtomographs designed to perform absorption microtomography and, by varying the sample-to-detector distance, phase microtomography (2 in the experimental hutch and 1 in the monochromator hutch for long propagation sample to detector distance: 13m!)
• A laminograph
• A grating interferometer
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. These imaging 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 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. ID16 is dedicated to this nanotomography project.
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 2000) 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 50 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 less than one second (“Ultrafast 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” or Paganin approach).