Measurement of the Parametrized Single-Photon Response Function of a Large Area Picosecond Photodetector for Time-of-Flight PET Applications

We have used a subpicosecond laser with an output trigger synched to a 4-GHz oscilloscope sampling at 25 GSa/s to characterize the temporally resolved response of a large area picosecond photodetector (LAPPD) to single photons/photoelectrons. The LAPPD is a large area (20 cm
$\times20$
cm) photosensor with a nanosecond-width time response arising from its underlying multichannel plate photomultiplier tube (MCP-PMT) structure. We used optical filters to attenuate the laser pulse intensity to a single-photon level. LAPPD pulses are nearly identical in shape, particularly for pulses whose peak amplitude is a large multiple of our system’s 0.6-mV rms baseline electronic noise. Three waveform timing algorithms were compared in their measurements of the transit time spread (TTS) Gaussian fitted to the distribution of measured single-photoelectron (SPE) pulse times under zero-width laser excitation: 1) a constant fraction algorithm; 2) a pulse centroid timing algorithm; and 3) a pulse model-fitting algorithm. The measured SPE values with the three algorithms are: [79±17, 67±15, and 82±20] ps Gaussian sigma, respectively. The LAPPD provides independent voltage control for its photocathode (PC), and at the entry and entry faces of its chevron pair of MCPs. We found no or minimal change in TTS with PC voltage for pulses >15 mV.