High resolution photon timing with MCP-PMTs: A comparison of a commercial constant fraction discriminator (CFD) with the ASIC-based waveform digitizers TARGET and WaveCatcher

There is considerable interest to develop new time-of-flight detectors using micro-channel-plate photomultiplier tubes (MCP-PMTs). The question we pose in this paper is whether available waveform digitizer ASICs, such as the WaveCatcher or TARGET, operating with a sampling rate of 2-3 GSa/s, can compete with 1GHz BW CFD/TDC/ADC electronics. We have performed a series of measurements with these waveform digitizers connected to MCP-PMTs operating at low gain and with a signal equivalent to ~40 photoelectrons. These tests were performed using a laser diode to illuminate the photodetectors under conditions comparable to those used in previous SLAC and Fermilab beam tests. Our measurement results indicate that one can achieve similar timing resolution with both methods. Although commercial CFD-based electronics are readily available and perform very well, they are impractical for large scale systems. In contrast, ASIC-based waveform recording electronics are well-suited to such applications, and do not require analog delay lines that otherwise make CFDs difficult to incorporate in ASIC designs.