Workshop for NSLS-II, March 2004

Summary - SAXS/XPCS Breakout Session

The SAXS/XPCS breakout session continued a discussion with the soft matter and biophysics user community that was initiated at the September 2003 workshop on "Scientific Opportunities in Soft Matter and Biophysics at NSLS-II." There were 23 participants in the breakout session representing 9 universities and 2 national labs.

The breakout session discussions focused on three x-ray techniques that will be significantly enhanced by the higher brightness of the NSLS-II source: time-resolved small-angle x-rays scattering (SAXS), x-ray photon correlation spectroscopy (XPCS), and grazing incidence small angle scattering (GISAXS).

The time-resolved SAXS discussion began with a talk by Tobin Sosnick on "SAXS and collapse in protein folding". This talk emphasized that, in addition to a higher brightness source, extending time-resolved SAXS to probe faster dynamics would require the development of area detectors with faster read-out times, improved micro-fluidics solution mixing chambers, and improved beamline component design to reduce extraneous background scattering. In the follow-up discussion, moderated by Lin Yang, it was proposed that background scattering could be reduced by minimizing, or even better, eliminating windows and optical elements upstream of the sample. This raised the question of whether it was feasible to design an undulator source with a single strong narrow peak in its spectrum. It was also suggested that, to meet the needs of the growing SAXS community, automated sample handlers would be necessary in order to reduce the overhead in the changeover between samples. Such instrumentation would also be useful for establishing mail-in type of sample characterization service, much like what is currently available in the protein crystallography community. Gas proportional area detectors were suggested as potential low-noise, photon-counting detectors. It was noted, however, that the development of fast, parallel read-out electronics was required. A final comment highlighted the need to co-develop bend-magnet as well as undulator beamlines since many time-resolved SAXS measurements were flux rather than brightness limited.

The discussion then focused on the technique of XPCS with an introductory talk by Robert Leheny on "XPCS and the slow dynamics in clay gels". This talk provided an example of a problem addressed using current state-of-the-art procedures in the field. In the follow-up discussion, moderated by Simon Mochrie, it was suggested that a more optimal beamline design for XPCS would incorporate a larger sample to detector distance than used at existing facilities. This has two advantages: (1) better angular resolution for the same size beam and (2) fewer photons per pixel. It was also suggested that the XPCS community consider the feasibility of developing larger-sized area detectors to simultaneously cover small and wide-angle x-ray scattering as has been done for neutron scattering. The proposed larger sample to detector distance impacts the overall length of the beamline and possibly the design of the NSLS-II building. For this reason, it was felt imperative to form a working group to evaluate how to optimize beamline design for preserving beam coherence and reaching the ultra-small angle scattering limit.

The final topic of discussion involved the techniques of GISAXS and grazing incidence diffraction (GID) with a lead-off talk by Detlef Smilgies on "GISAXS: a versatile tool for the study of structure and kinetics of organic thin films on nanoscopic length scales". This talk emphasized the importance of GISAXS and GID for the emerging field of nanoscience where researchers are producing assemblies in which properties are often dominated by structural changes at buried interfaces. In the follow-up discussion, moderated by Ben Ocko, it was noted that a key technique enhancement would be to incorporate vertical nano-focusing optics in a GISAXS/GID set-up. This would reduce the beam footprint on the sample minimizing the 'parallax problem' associated with diffraction from an extended source and enabling smaller sample areas to be probed.

A recurring theme in all of the discussion sessions was that the technical developments in sample-handling methods and x-ray detectors, which the x-ray scattering community agrees are crucial for effective use of the high-brightness NSLS-II source, would also offer significant improvements in experiments being done at the existing NSLS so should be a high funding priority.

Last Modified: February 3, 2012
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