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Figure 4
(a) A conversion from ADC units to exact photon numbers is made by binning the ADC units according to the distribution observed in a pulse-height histogram. Dead counts are effectively suppressed to ≤0.1 photon s−1 by adjusting a user-selectable threshold; the solid red region of the pulse-height histogram is therefore set to zero during the conversion process. The Poisson character of the acquired histograms is verified by an exemplary Poisson fit, plotted in light blue. An average number of [\overline{N}] = 2.4 photons pulse−1 is determined. (b) Pulse-height histograms obtained for two different X-ray intensities. Individual photon numbers can clearly be distinguished up to N = 20 photons pulse−1; the integral of the respective histograms is proportional to the incident X-ray flux up to N ≃ 120 photons pulse−1 (limited by the full-scale deflection of the ADC).

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