The Coherent Hard X-ray (CHX) beamline at NSLS-II will be dedicated to studies of nanometer-scale dynamics in materials using x-ray photon correlation spectroscopy (XPCS), as well as to other experimental methods enabled by bright, coherent, x-ray beams. XPCS is based on measuring time correlation functions of the speckle fluctuations that occur when a coherent x-ray beam is scattered from a disordered sample. It can be used to measure equilibrium dynamics via the “usual” single- speckle intensity-intensity autocorrelation functions g(2)(q, t). When combined with 2D area detectors and a multispeckle detection technique, it can also be used to measure non-stationary, non-equilibrium dynamics via two-time correlation functions g(2)(q, t1, t2). Higher-order correlation functions g(n)(q, t) can be used to characterize heterogeneities in the dynamical properties.
The key quantity that enables XPCS experiments is the source brightness. This determines the flux of coherent x-ray photons and ultimately the signal-to-noise ratio (SNR) of the measured correlation functions. With the unprecedented brilliance of the NSLS-II storage ring exceeding, for a photon energy near E~8 keV, 1021 ph/s/mrad2/mm2/0.1 % bw (more than one order of magnitude higher than that of the Advanced Photon Source), the CHX beamline will allow studies of dynamics on time scales that can be ~102=100 times faster and on shorter length scales than was ever possible before.
The design of the CHX instrument is simple and robust and puts an emphasis on three key elements: