Online digital data processing for the T2K fine grained detector

The T2K Fine Grained Detector is an active neutrino target that uses segmented scintillator bars to observe short-range particle tracks. 8448 multi-pixel photon counters coupled to wavelength shifting fibres detect scintillator light. An application specific integrated circuit shapes the MPPC waveform and uses a switched capacitor array to store up to 511 analog samples over 10.24μs. High and low attenuation channels for each MPPC improve dynamic range. 12-bit serial quad-ADCs digitize ASIC analog output and interface with a field programmable gate array, while each FPGA simultaneously reads out four ADCs and saves the synchronized samples in an external digital memory. The system produces 13.5 MB of uncompressed data per acquisition with a target trigger rate of 20 Hz, and requires zero suppression to reduce data size and readout time. Firmware based data compression uses an online pulse-finder that decides whether to output pulse height information, a section of waveform, or to suppress all data. The front end FPGA transfers formatted data to collector cards through a 2 Gb/s optical fiber interface using an efficient custom protocol. We have evaluated the performance of the FGD electronics system and the quality of its online data compression through the course of a physics data run.