Continuous timing measurement using a data-streaming DAQ system

We have investigated a prototype triggerless data acquisition (DAQ) system for charmed-baryon spectroscopy at J-PARC. A special time-to-digital converter (TDC) dedicated to continuous timing measurement without any external trigger was developed. A high dynamic range of the order of 1010 for the timing measurement was realized by introducing the heartbeat method, which periodically generates a delimiter datum. DAQ software receiving and filtering the data stream from the TDCs was also developed based on a message-queue application programming interface (API), FairMQ. We tested the prototype DAQ system with detectors using electrons and positrons converted from a bremsstrahlung photon beam with a rate condition of up to 1.9 MHz/1-mm detector segment. The DAQ received a 5.4 Gbps data stream from six TDCs and merged them to a complete data set. After sorting a certain portion of the data, the software searched for a set of corresponding data fragments in response to the penetration of a positron or electron using time separation between detector hits in the time axis and removed unnecessary hits. Around 25 GB RAM and 25 CPU threads were required for the online computing power under the test conditions. No silent data drop in the DAQ software was observed.