A Linear Time-Over-Threshold Digitizing Scheme and Its 64-channel DAQ Prototype Design on FPGA for a Continuous Crystal PET Detector

We propose a practical general method, called the linear time-over-threshold (linear TOT) scheme, for nuclear pulse amplitude digitization aiming at providing a high degree of integration for a high-channel count detector system. The most critical technique in the scheme is that for a different particle detector and diverse pulse shaping, how to produce the time-varying dynamic threshold voltage, which enables the linear conversion from the peak amplitude of nuclear pulse to the TOT time width. Compared with the normal time-over-threshold method for pulse height digitizing, the linear TOT has a strict linearity, large measurable signal dynamic range, and good signal-to-noise ratio. Using the scheme, we design a prototype of a field-programmable gate-array (FPGA)-based 64-channel data-acquisition (DAQ) system for a continuous crystal poistron emission tomography detector. The system only consists of 64-channel active RC integrators and voltage comparators, a shared dynamic threshold generating circuit, and an FPGA. The physical size of the system is so small that it could be attached on the base of the PMT. The preliminary test results show that the nonlinearity between pulse amplitude and TOT time width is about 0.01%, and the prototype could achieve an energy resolution of 12.3% by coincident spectrum measurement, which is better than the result (14.6%) obtained using our previous homemade DAQ system designed with a normal analog-to-digital converter technique. The design proves that the linear TOT scheme is an effective solution for a high-channel count digitizing system in nuclear detection, not only for its high performance, but also for a high degree of integration.