Experimental 3D coherent diffractive imaging from photon-sparse random projections.

The routine atomic resolution structure determination of single particles is expected to have profound implications for probing structure-function relationships in systems ranging from energy-storage materials to biological molecules. Extremely bright ultrashort-pulse X-ray sources - X-ray free-electron lasers (XFELs) - provide X-rays that can be used to probe ensembles of nearly identical nanoscale particles. When combined with coherent diffractive imaging, these objects can be imaged; however, as the resolution of the images approaches the atomic scale, the measured data are increasingly difficult to obtain and, during an X-ray pulse, the number of photons incident on the 2D detector is much smaller than the number of pixels. This latter concern, the signal 'sparsity', materially impedes the application of the method. An experimental analog using a conventional X-ray source is demonstrated and yields signal levels comparable with those expected from single biomolecules illuminated by focused XFEL pulses. The analog experiment provides an invaluable cross check on the fidelity of the reconstructed data that is not available during XFEL experiments. Using these experimental data, it is established that a sparsity of order 1.3 × 10-3 photons per pixel per frame can be overcome, lending vital insight to the solution of the atomic resolution XFEL single-particle imaging problem by experimentally demonstrating 3D coherent diffractive imaging from photon-sparse random projections.

Laboratory-based multi-modal X-ray microscopy and micro-CT with Bragg magnifiers.

We report on the successful demonstration of X-ray phase contrast microscopy and micro computed tomography (CT) with a Bragg magnifier microscope (BMM) in a laboratory setup. The Bragg magnifiers, constituted by two channel-cut crystals in asymmetric diffraction, produced a 15X magnification of the X-ray beam, thus enabling high resolution imaging to be attained. The angular sensitivity of the crystals was used to implement analyzer-based phase contrast imaging: acquiring images at different angular positions and the three parametric images (apparent absorption, differential phase and scattering) have been obtained. Micro-CT, with resolution of about 5 µm is demonstrated with the same system. The main limitations, as well as the ways to mitigate them, are discussed with the aid of the experimental data. The technique demonstrated herein extends high-resolution, multi-modal, x-ray imaging and micro-CT to compact laboratory setups, with the potential of broadening the reach of these techniques outside the community of synchrotron users.

X-ray Bragg magnifier microscope as a linear shift invariant imaging system: image formation and phase retrieval.

We present the theoretical description of the image formation with the in-line germanium Bragg Magnifier Microscope (BMM) and the first successful phase retrieval of X-ray holograms recorded with this imaging system. The conditions under which the BMM acts as a linear shift invariant system are theoretically explained and supported by the experiment. Such an approach simplifies the mathematical treatment of the image formation and reconstruction as complicated propagation of the wavefront onto inclined planes can be avoided. Quantitative phase retrieval is demonstrated using a test sample and a proof of concept phase imaging of a spider leg is also presented.

Potential use of V-channel Ge(220) monochromators in X-ray metrology and imaging.

While channel-cut crystals, in which the diffracting surfaces in an asymmetric cut are kept parallel, can provide beam collimation and spectral beam shaping, they can in addition provide beam compression or expansion if the cut is V-shaped. The compression/expansion ratio depends in this case on the total asymmetry factor. If the Ge(220) diffraction planes and a total asymmetry factor in excess of 10 are used, the rocking curves of two diffractors will have a sufficient overlap only if the second diffractor is tuned slightly with respect to the first one. This study compares and analyses several ways of overcoming this mismatch, which is due to refraction, when the Cu Kα1 beam is compressed 21-fold in a V21 monochromator. A more than sixfold intensity increase was obtained if the matching was improved either by a compositional variation or by a thermal deformation. This provided an intensity gain compared with the use of a simple slit in a symmetrical channel-cut monochromator. The first attempt to overcome the mismatch by introducing different types of X-ray prisms for the required beam deflection is described as well. The performance of the V-shaped monochromators is demonstrated in two applications. A narrow collimated monochromatic beam obtained in the beam compressing mode was used for high-resolution grazing-incidence small-angle X-ray scattering measurements of a silicon sample with corrupted surface. In addition, a two-dimensional Bragg magnifier, based on two crossed V15 channel monochromators in beam expansion mode and tuned by means of unequal asymmetries, was successfully applied to high-resolution imaging of test structures in combination with a Medipix detector.