The Large Area Picosecond Photodetector (LAPPD) is a microchannel-plate photomultiplier tube (MCP-PMT) of approximate size 20 cm × 20 cm with a capacitively coupled sensing electrode.
We measured the response of two LAPPD models, different in geometry and materials, to low-intensity picosecond laser pulses, mostly consisting of single photon hits. We investigated the timing distribution of signals and charge sharing on the segmented sensing electrode. The dominant prompt peak in time exhibits a width of $\sigma \sim 30 \>\mathrm{ps}$, where the full response is explained with a kinematic model of photoelectron trajectory and backscattering on the MCP surface. We developed analytical models of photoelectron propagation and induction of charge on the sensing electrode, the latter incorporating elastic back-scattering of the multiplied charge.
The model reproduces the timing and spatial properties of the measured signals very well, and they significantly depend on the sensor geometry and materials. The results of this work are useful in the optimisation of MCP-PMTs for various applications, such as RICH detectors and TOF-PET systems.
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