Scientific Reviews
transferred into real-world applications was recently raised after some XPCi images obtained with an extended source were presented [17]. However, these images were obtained only at the price of artificially increasing the source spatial coherence by strongly aperturing its output through a third grating, placed in contact with the source itself. This approach is well known in optics, and corresponds to switching form a Talbot to a Talbot-Lau configuration.The problem in x-ray imaging applications is that, by doing this, one suppresses the source output, leading to exposure times. In fact, an equivalent approach is obtained by switching back to the Talbot configuration (i.e. eliminating the source grating) and using a microfocal source, as discussed in [18].
Figure 1. Pixel edge illumination concept (see text.).
RELAXED COHERENCE CONDITIONS: REFRACTION-BASED METHODS
The common reason why the above methods cannot currently be transferred into real-world applications is that they are both based on detecting and/or generating interference patterns. This requires the source to be spatially coherent, and there does not exist at the moment a source simultaneously featuring a sufficiently small focal spot and high emitted power to allow the acquisition of XPCi images within reasonable timescales. An alternative approach relies on the small angular deviations that x-rays undergo when crossing an object as a consequence of phase distortions, i.e. on x-ray refraction. This is described by the simplified ray-optics model, in which the direction of the photon (seen as a ray) is locally orthogonal to the wavefront (e.g. a plane wavefront corresponds to parallel x-rays all travelling in the same direction). Crossing a non-homogeneous object corresponds to imprinting different shifts to different parts of the wavefront, thus distorting it. These distortions translate into local deviations from the original photon direction. It is important to note that this is not a different phenomenon from those described previously, which can only be rigorously described by wave-optics: this is a simplified description, that holds under relaxed coherence conditions. The main message here is that phenomena that can be described with this model still exist under relaxed coherence conditions. Therefore, unlike fine interference fringes, it will still be possible to detect them while using a radiation source with much less coherence. For a discussion of the conditions 13
Notiziario Neutroni e Luce di Sincrotrone - Vol. 15 n. 2