Cosmic ray Cherenkov and fluorescence imaging: Photosensors and data acquisition systems for a new generation of focal planes

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In this paper the design of a new generation of focal planes for Imaging Atmospheric Cherenkov and Fluorescence Telescopes is discussed, based on a Digital Photon Counting architecture instrumented with (SiPM) Silicon Photomultipliers. 3 × 3mm2 SiPMs were chosen to minimize the photoelectron pile-up within intervals shorter than the clock sampling time. The large number of channels requires a compact, modular design with minimal cabling and distance between the photosensors and the frontend electronics. Other design requirements are a high reliability, easy field maintenance and minimization of the total power budget. Data acquisition electronics are partitioned in on-board frontend and off-detector high-level trigger electronics. Extensive use of mixed-signal ASICs and low-power FPGAs for early data reduction were adopted. Temperature is controlled by a liquid cooling sub-system. An asynchronous data readout and filtering, where each of the trigger levels works at its own clock frequency is adopted. The off-detector data acquisition and trigger architecture is based on an asynchronous multi-gigabit switching approach implemented over standard MicroTCA boards, equipped with optical interfaces and high-capability FPGAs. The boards are connected by multi-Gbps optical links to the frontend electronics. Trigger primitives are sent asynchronously to the MicroTCA trigger boards via data links running at their own clocks, for maximum bandwidth. Data and slow-control data streams are sent over the same data lines, reducing in this way the number of cables required. Each crate can process up to 4 × 104 channels and the modularity offered allows further expansion by inter-connecting additional crates with optical fiber links.