Characterisation of the LAPPD, a large area microchannel-plate PMT

TL;DR

This work characterises the Gen-II LAPPD MCP-PMT with a capacitively coupled external readout, focusing on single-photon timing and spatial response. The authors perform precision bench measurements on two detector variants and develop a physics-based model for photoelectron propagation and secondary-electron induced signals using Shockley–Ramo theory, including elastic backscattering effects. The model reproduces the observed timing distributions and pad-charge sharing across geometries, enabling extrapolation to different back-plate materials and gaps. The results provide a predictive framework to optimize MCP-PMT designs for timing- and imaging-critical applications such as RICH detectors and TOF-PET, and outline concrete directions for custom readouts and detector-simulation integration.

Abstract

We present a comprehensive characterization of the LAPPD Gen-II, a large-area microchannel-plate photomultiplier tube (MCP-PMT) equipped with a capacitively coupled sensing (readout) electrode. Two detector variants with different geometries and materials were investigated using a picosecond pulsed laser system. We measured the single-photon timing response and spatial charge distribution on segmented readout electrodes. The prompt timing peak exhibits a resolution of approximately 30 ps, with the overall timing structure explained by photoelectron propagation and back-scattering effects from the MCP input surface. We developed analytical models that describe the propagation of photoelectrons and the induced charge spread on the sensing electrodes, including secondary electron backscattering from the resistive anode. The model accurately reproduces the measured device properties and enables performance extrapolations for various detector geometries and dielectric properties. These results provide a predictive framework for optimizing MCP-PMTs for timing- and imaging-critical applications such as RICH detectors in high-energy physics and TOF-PET systems for medical imaging.

Figures (19)

• Figure 1: LAPPD sensor cross-section: schematic layout of the LAPPD detector with a definition of the main potential levels, geometric parameters, and dielectric constants (left); photo of the front face of the LAPPD detector (right).
• Figure 2: External read-out electrode, the original (Incom) sensing electrode with pads at 25.4 mm pitch.
• Figure 3: The LAPPD detector in the measurement setup; the laser light propagates through the yellow fibre to the focuser (outlined in green) that moves with the 3D stages (outlined in blue) to the photocathode window (outlined in magenta). The red cables connect the MCPs to the power supply for biasing, and the ribbon cable connects to the Z stage.
• Figure 4: Time-walk correction: signal timing as a function of signal charge raw data with the correction curve (in red, $f(ADC)=-612\>\rm{ps} + 8166\>\rm{ps}/\sqrt{ADC\left[channels\right] + 13.97}$).
• Figure 5: The measured timing distribution of pulses in LAPPD #162 after time walk correction for different potential differences between the photocathode and the microchannel plate 1 ($U_{PC-MCP1in}$). The result of the fit with a sum of two Gaussian functions with parameters $A_i, \mu_i, \sigma_i$ is shown in red, and the fitted parameters are displayed in the box in the top right corner of each of the plots; the dashed lines indicate the peak of the prompt peak and the end of the approximately flat component of the time spectrum ($t_{max}$).