X-ray speckle visibility spectroscopy in the single-photon limit

Abstract

The technique of speckle visibility spectroscopy has been employed for the measurement of dynamics using coherent X-ray scattering. It is shown that the X-ray contrast within a single exposure can be related to the relaxation time of the intermediate scattering function, and this methodology is applied to the diffusion of 72 nm-radius latex spheres in glycerol. Data were collected with exposure times as short as 2 ms by employing a resonant shutter. The weak scattering present for short exposures necessitated an analysis formalism based on the spatial correlation function of individual photon charge droplets on an area detector, rather than the usual methods employed for intensity correlations. It is demonstrated that this method gives good agreement between theory and experiment and thus holds promise for extending area-detector-based coherent scattering methods to the study of faster dynamics than previously obtainable.

Low Dose X-Ray Speckle Visibility Spectroscopy Reveals Nanoscale Dynamics in Radiation Sensitive Ionic Liquids

“…flow cells [11] or changing the exposure spot on the sample. However, low dose XPCS experiments often show very noisy correlation functions which do not yield conclusive insights into the dynamics or did not extend beyond feasibility studies [12,13].…”

Hard-sphere-like dynamics in highly concentrated alpha-crystallin suspensions

“…The 7.34 keV x-ray beam was focused vertically using a Be compound refractive lens to 5.24 µm (FWHM), in order to increase the signal to noise ratio for XPCS. 16 Slits fixed the horizontal beamsize to 20 µm. The incident x-ray flux within the illuminated volume was 6 × 10 10 photons/s.…”

Radiation damage limits to XPCS studies of protein dynamics

“…Here β is the optical contrast and q the scattering vector. Alternately, one can measure the persistence of speckle directly through the spatial autocorrelation of an image measured with a finite exposure time, τ and then vary the exposure time [2]. In this case one obtains a visibility function V(τ) whose value approaches 1 + β at short times and 1 at long times.…”

“…The speckle persistence can be measured via time autocorrelation of the scattering intensity, within a region comparable to the size of a speckle. One defines a time autocorrelation function by g 2 (τ) = I(q, t)I(q, t + τ) / I(q) 2 . For the case of simple diffusive motion with diffusion constant, D, one can show that g 2 (τ) = 1 + βe −2Dq 2 τ .…”