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Proceedings Paper

Analysis of the optical design of the NSLS-II coherent hard x-ray beamline
Author(s): Andrei Fluerasu; Oleg Chubar; Konstantine Kaznatcheev; Jana Baltser; Lutz Wiegart; Kenneth Evans-Lutterodt; Mary Carlucci-Dayton; Lonny Berman
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Paper Abstract

Ultra-low emittance third-generation synchrotron radiation sources such as the NSLS-II offer excellent opportunities for the development of experimental techniques exploiting x-ray coherence. Coherent light scattered by a heterogeneous sample produces a speckle pattern characteristic for the specific arrangement of the scatterers. This may vary over time, and the resultant intensity fluctuations can be measured and analyzed to provide information about the sample dynamics. X-ray photon correlation spectroscopy (XPCS) extends the capability of dynamic light scattering to opaque and turbid samples and extends the measurements of time evolution to nanometer length scales. As a consequence XPCS became crucial in the study of dynamics in systems including, but not being limited to, colloids, polymers, complex fluids, surfaces and interfaces, phase ordering alloys, etc. In this paper we present the conceptual optical design and the theoretical performance of the Coherent Hard X-ray (CHX) beamline at NSLS-II, dedicated to XPCS and other coherent scattering techniques. For the optical design of this beamline, there is a tradeoff between the coherence needed to distinguish individual speckles and the phase acceptance (high intensity) required to measure fast dynamics with an adequate signal-to-noise level. As XPCS is a "photon hungry" technique, the beamline optimization requires maximizing the signal-to-noise ratio of the measured intensity-intensity autocorrelation function. The degree of coherence, as measured by a two-slit (Young) experiment, is used to characterize the speckle pattern visibilities. The beamline optimization strategy consists of maximization of the on-sample intensity while keeping the degree of coherence within the 0.1-0.5 range. The resulted design deviates substantially from an ad-hoc modification of a hard x-ray beamline for XPCS measurements. The CHX beamline will permit studies of complex systems and measurements of bulk dynamics down to the microsecond time scales. In general, the 10-fold increase in brightness of the NSLS-II, compared to other sources, will allow for measurements of dynamics on time-scales that are two orders of magnitude faster than what is currently possible. We also conclude that the common approximations used in evaluating the transverse coherence length would not be sufficiently accurate for the calculation of the coherent properties of an undulator-based beamline, and a thorough beamline optimization at a low-emittance source such as the NSLS-II requires a realistic wave-front propagation analysis.

Paper Details

Date Published: 23 September 2011
PDF: 7 pages
Proc. SPIE 8141, Advances in Computational Methods for X-Ray Optics II, 81410J (23 September 2011); doi: 10.1117/12.894129
Show Author Affiliations
Andrei Fluerasu, Brookhaven National Lab. (United States)
Oleg Chubar, Brookhaven National Lab. (United States)
Konstantine Kaznatcheev, Brookhaven National Lab. (United States)
Jana Baltser, Univ. of Copenhagen (Denmark)
Lutz Wiegart, Brookhaven National Lab. (United States)
Kenneth Evans-Lutterodt, Brookhaven National Lab. (United States)
Mary Carlucci-Dayton, Brookhaven National Lab. (United States)
Lonny Berman, Brookhaven National Lab. (United States)

Published in SPIE Proceedings Vol. 8141:
Advances in Computational Methods for X-Ray Optics II
Manuel Sanchez del Rio; Oleg Chubar, Editor(s)

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