Influence of the threshold settings of photon counting detectors on the observed speckle contrast in coherent scattering experiments

Speckle-based experiments, in particular X-ray Photon Correlation Spectroscopy (XPCS), are among the ones that benefit most from the development of next generation light sources. The key quantity that determines whether or not it will be possible to perform an XPCS experiment is the speckle contrast β, which measures the visibility of the speckle pattern. The speckle contrast is influenced by geometrical conditions and light-source properties [1], and recently it has been noticed that artifacts in photon-counting detectors strongly interfere with the determination of the real value of β when the number of detected photons per frame is extremely low [2]. Here we report how the threshold setting of a photon counting detector, an EIGER X 4M, affects the detected contrast even at non extreme illumination conditions (0.01-0.05 photons/pixel/second). We found that by increasing the threshold value, not only leads to the expected drop in detected intensity, but also to a significant increase in the value of β. Additionally we will present an in-operando situation in which the higher threshold helps critically to increase the signal-to-noise ratio of the measured scattering pattern resulting in a five-fold increment of the speckle contrast.