Analysis of the optical design of the NSLS-II Coherent Hard X-ray beamline
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.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- USDOE SC OFFICE OF SCIENCE (SC)
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1048194
- Report Number(s):
- BNL-98128-2012-CP; 39KC02000; TRN: US201216%%790
- Resource Relation:
- Conference: Advances in Computational Methods for X-Ray Optics II; San Diego, California, United States ; 20110821 through 20110825
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ALLOYS
APPROXIMATIONS
BRIGHTNESS
COHERENCE LENGTH
COHERENT SCATTERING
COLLOIDS
DESIGN
FLUCTUATIONS
LIGHT SCATTERING
MODIFICATIONS
OPTICS
OPTIMIZATION
PERFORMANCE
PHOTONS
POLYMERS
SIGNAL-TO-NOISE RATIO
SPECTROSCOPY
SYNCHROTRON RADIATION SOURCES
exploiting
scatterers
spectroscopy
nanometer
national synchrotron light source