Scientists have been using both I13 and ID17 at the ESRF, Grenoble, France to develop a new quantitative x-ray phase-contrast imaging method, based on the edge illumination principle, which achieves unprecedented nanoradian sensitivity. Using both very high and very low x-ray energies at the two facilities, the team showed that this highly sensitive technique can be efficiently exploited over a very broad range of experimental conditions. Not only that, it is simple, scalable, and relatively insensitive to mechanical and thermal instabilities. The team hope that the method will open the way to new, previously inaccessible scientific applications in biology, medicine and materials science. Their research has been published in Physical Review Letters.
Phase contrast imaging offers significant better contrast than conventional absorption imaging, which is and is particularly important for materials made of light elements and when high energy X-rays are needed. As a result, in recent years a lot of research has gone into developing new methods. Several approaches have been tried including crystal interferometry, analyzer-based imaging, free-space propagation, grating interferometric and non-interferometric methods. The group used Diamond and ESRF to explore a new quantitative non-interferometric method that pushes the limits of sensitivity beyond existing techniques by at least an order of magnitude.
The method is underpinned by the principle of edge illumination. This involves narrowing down the x-ray-beam illuminating the sample, placing an absorbing edge in front of the detector and analyzing the transmitted beam on a thin line of detector pixels. The sample is then scanned through the beam in the direction orthogonal to the edge. This generates an image characterized by bright and dark fringes around the object edges, in addition to the absorption signal visible in the bulk regions of the sample.
Recently an algorithm has been developed which makes it possible to differentiate between the absorbed and refracted beam. However, this assumes that the incident beam on the detector is the geometric projection of the pre-sample aperture. For highly coherent x-ray beams, and where there is a large gap between sample and detector, it is necessary to take wave diffraction effects into account. This allows the method to be adapted to the irregular beam shape provided by highly coherent synchrotron sources and large propagation distances, achieving extremely high angular sensitivity.
The method was tested on both ID17 at the ESRF, Grenoble, France and on the Coherence branch of I13 at Diamond. The results demonstrated an improvement of at least an order of magnitude in sensitivity over previous experiments. At Diamond in order to demonstrate the extremely small signals detectable with this setup, an even more challenging sample was imaged: a 10 µm thick polypropylene film in water. Despite the very weak signal provided by this sample, the edge of the film is clearly visualized. The faint signal provided by this object, as well as its dimensions, is similar to that expected from a single cell in a liquid environment. The obtained result, therefore, opens the way to new applications of X-rays in the imaging of single cells in biological samples.
Performance and characterization of the prototype nm-scale spatial resolution scanning MLL microscope Journal Paper, Evgeny Nazaretski,Jungdae Kim,H Yan,Kenneth Lauer,D Eom,Y S Chu,D Shu,J Maser,Zoran Pesic,Ulrich Wagner,Christoph Rau, Review Of Scientific Instruments 2013
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