Network Time Synchronization for Detector Data Acquisition Electronics

Large scale nuclear physics experiments often use arrays of radiation detectors, which can be physically separated, for example in separate rooms or along a beamline. Time synchronization of the detector readout electronics, which is essential to detect related events, is often accomplished by sharing clock, trigger and reset signals within and between racks of digitizing electronics. This works well over short distances, but requires dedicated cabling and/or modules and becomes cumbersome for widely separated arrays. As the detector readout electronics is in many cases operated by computers linked over standard data networks, an alternative to dedicated clock distribution trees is the synchronization of clocks over the network. Network time synchronization protocols like IEEE 1588 or White Rabbit are reported to achieve low/sub nanosecond timing resolution, but these results generally refer to clock offsets computed in software or to once-per-second reference pulses, not to the capture of detector pulses with amplitudes and waveforms.The work reported here thus focuses on integration of network time synchronization techniques with digital data acquisition for radiation detectors. Detector waveforms are captured with analog to digital converters that are synchronized to the network master clock and/or are tagged with time stamps related to the network time. Key to this integration is the handling of network clock and time in the same FPGA firmware that processes the analog to digital converter's data stream. We tested implementations based on FPGA firmware and on network hardware, and report timing resolutions obtained for two synchronized detectors capturing coincident radiation.