A large area Ring-Imaging Cherenkov detector has been designed to provide clean hadron identification capability in the momentum range from 3 GeV/c up to 8 GeV/c for the CLAS12 experiment. The adopted solution foresees a novel hybrid optics design based on aerogel radiator, composite mirrors and high-packed and high-segmented photon detector.
The photon detector must efficiently detect single photons in the visible light region, provide a fast response for background rejection and pattern recognition, have a spatial resolution of less than 1 cm covering an area of about 1 m2. A first sector has been recently put into operation using for the first time the well known Hamamatsu H8500 MAPMT and the new single photon dedicated H12700, for a total of 400 MA-PMTs and about 25000 pixels.
Spatial, thermal and low noise requirements led to design a modular front-end electronics capable of local signal processing and data buffering, optically linked to the central acquisition node. Each readout unit is composed by a group of MA-PMTs (2 or 3) and a stack of three electronic boards with specific tasks. The core of the readout is composed by the Multi Anode Read Out Chip (MAROC), able to discriminate the 64 signals from one MA-PMT and to produce 64 corresponding binary outputs with 100% efficiency starting at 50 fC injected charge level, and by the FPGA board that provides 1 ns TDC capability with 8 us maximum latency and acts as a controller and DAQ board. The system is designed to be almost dead-time free at the foreseen 20 kHz CLAS12 trigger rate.
Each unit has been validated and characterized for single-photon detection using external light sources (laser stand and online LED system), the on-board charge injector, thermal dark counts and real data from the CLAS12 engineering run. A cosmic stand has been realised to study the global performance of the detector before installation. The best working conditions, the optical and electronic cross-talk, time walk correction, channel-by-channel timing and equalization, dark count and noise level have been studied at laser stands and with the electron beam data. The detector started the physics data taking at the beginning of February 2018. A report of the photon detector preparation, commissioning and operation will be presented.