Rapid-Acquisition Fluctuation Electron Microscopy: Decoherence and the Dominant Role of Noise

We investigate how data acquisition rate affects the decoherence of diffraction speckles in fluctuation electron microscopy (FEM) experiments on amorphous silicon at 80 kV. Surprisingly, reducing acquisition time from 256 ms to 1 ms does not significantly enhance the intensity variance peaks related to medium-range order. This suggests that decoherence processes operate at timescales faster than 1 ms. At the highest acquisition rates, noise complicates the variance background estimation. A significant source of non-Poisson noise is the spread of electron signals across adjacent detector pixels. We mostly restore the discrete pulse counting needed to mitigate Poisson noise by rounding pixel intensity to the nearest integer-electron value. However, a residual negative-variance offset grows as the acquisition rate increases. Efficient electron pulse counting in detectors is crucial for processing Poisson noise in FEM, especially with weak signals.