Dr Michael Ziolkowski (University of Siegen)
Structural dynamics of crystals is a topic of high interest for modern X-ray crystallography. It is studied by observing Bragg diffraction peaks with X-ray- or synchrotron radiation under a dynamically applied external electric field perturbation of an arbitrary shape with repetition rates between a few Hz up to several MHz. The aim of this contribution is to describe our recent development of a novel compact FPGA-based *data acquisition (DAQ) system* for a 10-channel detector system, installed at the synchrotron facility PETRA III. Here, 10 avalanche photodiode X-ray photon point detectors are mounted on a movable diffractometer arm. While a single detector analyzes one narrow angular range at a time, requiring time consuming angular scans over the range of interest, a *multi-channel-detector setup*, in which each point detector is mounted at a slightly different angular position, can be used to cover a wider angular range and so enabling the observation of several diffracted peaks *simultaneously*. Using our DAQ-system, we determine the arrival times of diffracted photons relative to a common trigger signal for each of the 10 point detectors simultaneously. The trigger signal is generated at the beginning of each excitation period. To measure the arrival times of photons, a dedicated concept of *multi-channel time-to-digital converter* was designed and implemented in the Xilinx Spartan-6 FPGA platform by utilizing its very fast clock-synchronous serializer/deserializer elements. A temperature-independent time resolution of up to 2 ns was achieved. The measured arrival times of photons are subsequently processed and distributed between 10000 time-channels per point detector, each of them capable of storing up to 65535 photon counts. At each angle step, data is collected during the measurement time of typically several seconds. While the diffractometer motor moves to the next angular position, the collected data is transferred to a PC . A pipelined system architecture, combined with parallel event processing ensures *zero-dead-time data acquisition* and enables a photon counting rate up to 250 MHz per point detector. For experiments with low excitation repetition rates, a *re-binning-mode* of operation increases the time channel width to a multiple of 2 ns, while keeping the photon detection resolution and the total number of time-channels unchanged. The developed DAQ system is easily portable between facilities and scalable in terms of detector channels. It has recently been used successfully for our novel time-resolved X-ray diffraction experiments at PETRA III and European Synchrotron Radiation Facility.