Calibration of the non-linear system characteristic of the DSSC detector for the European XFEL

The DSSC (DEPFET Sensor with Signal Compression) is a new instrument with non-linear compression of the input signal and with parallel signal processing (filtering, linear amplification, and 8/9-bit digitization) for all pixels. The DSSC will serve as ultra-fast 2d megapixel imaging detector at the European XFEL (X-ray Free Electron Laser) currently under construction in Hamburg, Germany. The DSSC design goal is to achieve at the same time single photon detection and high dynamic range of about 10 4 photons, both for photon energies down to 0.5 keV and read-out speeds up to 4.5 MHz. Realization of this goal requires an accurate calibration of the non-linear system characteristic (NLSC) over the full dynamic range of the detector. The NLSC describes the relation between the signal charge collected in a sensor pixel and the respective digital output of the DSSC system, and must be determined for each of the 1024 × 1024 DSSC pixels. We report results from a set of measurements aimed at calibrating the NLSC of a single DSSC prototype pixel for photon energies of about 0.5 and 1 keV using 55 Fe as X-ray line source. In addition, a 109 Cd source was used for independent calibration and verification. For further independent verification, we compared the DSSC prototype results to measurements performed with an external test bench providing 14-bit resolution.