Fast Charge to Pulse Width Converter for Monolith PET Detector

Currently both preclinical and clinical PET systems are built with pixilated, optically isolated scintillators. The use of optical isolators limits the achievable packing fraction for designs that use small crystals. No optical isolation is necessary in a monolithic scintillation crystal design hence the sensitivity is increased. The light distribution created by a high energy interaction in a monolithic scintillator can be readout by a SiPM-array to determine the 3-D position of the interaction. We have developed a digital pulse width modulation readout circuit that is able to readout many densely packed SiPM-arrays connected to monolithic scintillators. A monolithic scintillation detector requires simultaneous acquisition of the light distribution on multiple sensors unlike in a one-to-one optically isolated pixelated configuration. Therefore, pulse width modulation can reduce the readout complexity of the monolithic scintillation detector. The circuit gives an output signal with a pulse width linear to the incoming charge. Therefore, the circuit provides both timing and intensity information using just one digital line per channel. The charge to pulse-width conversion ratio of the circuit is adjustable (e.g., 33 ns/pC). The trigger jitters σ 41.2 ps between two channels. The PCB offers 8 channels and comes with additional features: The gain variation of a SiPM is compensated over a large temperature range by controlling the bias voltage. We measured from 15°C to 42°C, here it can bring the variation of a SiPM in gain from -21 ns/ °C to +3.8 ns/ °C or stabilizes the variation of the energy resolution between 18 and 20%.