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Figure 3
Oscilloscope trace of various APD pulses generated by the Hamamatsu S12023-05 APD installed at the PolLux endstation when exposed to a 1 keV X-ray photon beam. The maximum amplitude of the APD pulse is marked by the red curve. A significant distribution of amplitudes can be observed but, importantly, the shape of the pulse does not change with its amplitude, and comprises a first symmetric peak followed by a lower voltage sinusoidal signal caused by the ringing of the APD/RF amplifier circuit. By selecting the threshold of the time-to-digital converter to be above this sinusoidal signal (e.g. the threshold defined by the magenta dashed line), we can reliably determine the center of the first pulse and extract the photon arrival time from it.

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