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Photon Sciences Directorate at Brookhaven National Laboratory

Beamline & Optics R&D: Enhancing Tools at NSLS, NSLS-II

Qun Shen

"Synchrotron sources have quickly become an essential tool for a wide spectrum of research. All the action takes place at beamlines, each one consisting of a suite of sophisticated scientific instruments. The robust beamlines at NSLS produce remarkable science, and we made excellent progress on developing NSLS-II beamlines and associated science programs." — Qun Shen Director, Photon Division

While keeping the existing ring and beamline mechanical systems running, Photon Sciences staff completed a number of R&D projects this year that will improve the tools of researchers at NSLS and, in the near future, NSLS-II.

One of the major accomplishments was the installation and commissioning of NSLS beamline X17A, which is optimized for x-ray total scattering and atomic pair distribution function (PDF) experiments. The new beamline will alleviate the large demand for beam time on high-energy beamlines X17B1, X17B2, and X17B3. The scientific focus of X17A will be the structural characterization of disordered, nanocrystalline and complex nanostructured bulk materials at ambient and extreme conditions. The beamline features a novel Laue diffraction monochromator design that uses two-axis bending of the silicon crystal to simultaneously focus the beam in two orthogonal planes. Due to the bending, lattice strain increases integrated reflectivity of the Laue diffraction crystal by an order of magnitude compared to a perfect crystal. Anticlastic bending, a natural phenomenon for a crystal under strain, is typically considered a nuisance that must be “controlled.” In this case, it allows meridional focusing. A bender design, one that holds a rectangular crystal at the corners and bends it at the edges, makes slight adjustments to the "natural" anticlastic curvature to give the two-dimensional focusing.

X17A monochromator

The X17A monochromator in the X17 front end, shown when rolled out of the chamber

Also this year, researchers developed a new method for measuring x-ray optics aberrations. Creating the very small x-ray spots used for synchrotron nanoscale research requires a specialized and nearly perfectly formed optic to modify the phase profile of the beam. To look for aberrations — errors in the phase profile due to the optics (which can broaden and distort the spots) — scientists usually scan a slab of metal called a knife edge across the beam face. But to measure the beam size accurately with this method, the knife edge must be smaller than the beam. This is a problem for the types of nanoscience experiments planned at NSLS-II, where the beam is so small that the knife edge would have to be just one atomic plane thick — much too thin to fabricate. Photon Sciences researchers, along with colleagues at Argonne National Laboratory, the University of Rochester, and Cornell University, have figured out how to use an alternative called phase retrieval. The technique allows researchers to scan the beam with much larger feature sizes and then mathematically reconstruct the data to characterize the beam and its aberrations at any point along its path.


Click on the image to download a high-resolution version.Top: Experimental setup of the phase retrieval technique. Bottom: Measured and simulated lens aberrations (left) and their corresponding through focus amplitude of reconstructions (right) for different lens angular misalignments. (From Manuel Guizar-Sicairos et al., Appl. Phys. Lett. 98, 111108 [2011])

Other achievements made in 2010 were the installation of a transmission x-ray microscope at beamline X8C and the use of a wedged multilayer Laue lens to reach a record level of x-ray nanofocusing (see sidebars), and the creation of a large scanning stage that allows researchers to take x-ray fluorescence images of hefty items like paintings in a very short time frame. Art historians have been particularly interested in the latter development as a way to test the authenticity and genesis of artwork.