Higher Brightness Coherent X-Ray Beams: New Possibilities for Soft Matter Studies
Sunil K. Sinha
University of California San Diego / LANSCE, Los Alamos National Laboratory
The potential of an ERL synchrotron X-ray source with brightness and time-averaged coherent flux 2-3 orders of magnitude greater than those currently available at third generation sources opens up new possibilities for scattering studies of condensed matter. Current soft condensed matter studies with X-rays are focused on the following techniques: small angle x-ray scattering (SAXS), often with extension also to wide-angle X-ray scattering (WAXS); X-ray reflectivity and off-specular scattering and Grazing Incidence Small Angle Scattering (GISAXS) for thin films and planar arrays of objects; and X-ray Photon Correlation Spectroscopy (XPCS) to study slow dynamical fluctuations. Scattering experiments with micro- and nanobeams of x-rays have also begun to flourish at 3rd generation sources. Of these, XPCS is still quite severely limited even at 3rd generation sources, while off-specular scattering, and to some extent WAXS are somewhat intensity limited. Nanobeam scattering and also time-dependent studies involving these techniques, e.g. pump/probe experiments would also be dramatically impacted by orders of magnitude higher flux. Finally, there are the recent developments in phase reconstruction and so-called lensless imaging of nano-objects using coherent beams and holography. We shall discuss the kinds of studies which are currently difficult to carry out, and which will be enabled by the ERL.
The impact of having beams which are mainly coherent will of course pose special challenges in the analysis of data from experiments such as SAXS, GISAXS and off-specular diffuse scattering which have been mainly carried out within the framework of the Distorted Wave born Approximation applied to scattering from incoherent beams. We shall discuss what the effects of coherence will have on the analysis of such experiments. We shall also discuss the complementary problem, namely what effects instrumental resolution limiting the coherence will have on the analysis of experiments which assume completely coherent beams, such as phase-reconstruction based imaging and XPCS. We shall also discuss the effects of coherent nanobeams produced, e.g. from an X-ray waveguide. Finally, we will attempt to explore what happens when we apply such techniques to quantum systems in coherent states.
