Abstract
X-ray speckle visibility spectroscopy using X-ray free-electron lasers has long been proposed as a probe of fast dynamics in noncrystalline materials. In this paper, numerical modeling is presented to show how the data interpretation of visibility spectroscopy can be impacted by the nonidealities of real-life X-ray detectors. Using simulated detector data, this work provides a detailed analysis of the systematic errors of several contrast extraction algorithms in the context of low-count-rate X-ray speckle visibility spectroscopy and their origins are discussed. Here, it was found that the finite detector charge cloud and pixel size lead to an unavoidable `degeneracy' in photon position determination, and that the contrasts extracted using different algorithms can all be corrected by a simple linear model. The results suggest that experimental calibration of the correction coefficient at the count rate of interest is possible and essential. Thisallows computationally lightweight algorithms to be implemented for on-the-fly analysis.
Highlights
X-ray photon correlation spectroscopy (XPCS) studies the dynamics of amorphous and disordered material systems via measuring the fluctuations of speckle patterns resulting from coherent X-ray scattering (Grubel et al, 2008)
The advent of X-ray free-electron lasers (FELs) opens up the possibility for capturing ultrafast atomic-scale dynamics with speckle visibility measurement by using a pair of femtosecond X-ray pulses with time separation ranging from femtoseconds to nanoseconds to define the ‘exposure time’ (Gutt et al, 2009; Stephenson et al, 2009; Emma et al, 2010; Altarelli, 2011; Ishikawa et al, 2012; Kang et al, 2017; Milne et al, 2017)
We used three different photon assignment algorithms to evaluate contrast of the simulated datasets, labeled: Greedy Guess (GG), Least Squares Fit (LSF) (Hruszkewycz et al, 2012), and Psana Photon Convertor (PPC) (Yoon, 2016; Thayer et al, 2017); below we present a brief description of each
Summary
X-ray photon correlation spectroscopy (XPCS) studies the dynamics of amorphous and disordered material systems via measuring the fluctuations of speckle patterns resulting from coherent X-ray scattering (Grubel et al, 2008). In order to probe the dynamics of interest without perturbing the system with the X-ray pulse itself, the singlepulse radiation dose must be limited This leads to one of the main challenges for speckle visibility measurements at X-ray FELs: the very low intensity of the speckle patterns (Hruszkewycz et al, 2012; Perakis et al, 2018; Roseker et al, 2018). Other experiments (Yoon, 2016; Perakis et al, 2018) adopted a faster algorithm that skips the step of ‘dropletizing’ and counts the photon numbers of each pixel directly from its readout based on a simpler model of charge cloud in order to extract visibility from speckle patterns. The inhomogeneity of the detector in terms of the pixel-to-pixel gain variation is modeled by multiplying the
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