Numerical Simulation on Magnetic Field Tolerance of MCP-PMTs

Microchannel plate photomultiplier tubes (MCP-PMTs) with high temporal resolution and low dark count rates will be used as single-photon detectors for the Super Tau-Charm Facility (STCF) proposed in China. Similar to other RICH or DIRC detectors, MCP-PMTs in the STCF need to operate properly in an axial magnetic field of about 1.5 T. We study the magnetic field tolerance of MCP-PMTs with the simulation methods based on the Furman model as a secondary electron emission model and the particle-in-cell method for tracking electrons. The effects of MCP structural parameters (aperture $D$ , length-to-diameter ratio $L/D$ , bias angle $\theta $ , and applied voltage $U$ ) on the magnetic field tolerance are simulated. The results show that the smaller the $D$ and the smaller the $L/D$ of the MCP, the better its magnetic field tolerance. By increasing the MCP bias angle, its magnetic field tolerance increases first and then weakens. The applied voltage does not affect its magnetic field tolerance. Changing the angle between the magnetic field and the normal direction to the microchannel plate (MCP), the gain decays fastest when the magnetic field direction is parallel to the axial direction of the MCP channels; at a magnetic field direction of 60°, the MCP gain decays the slowest. For MCP-PMTs, the change in the magnetic field direction causes the alteration of the motion direction of the electron cloud in the gaps (cathode-MCP1, MCP1-MCP2, and MCP2-anode), which is the principal reason for the difference in its gain in different magnetic field directions with the same magnetic field strength. The magnetic field tolerance of different assembly methods of two MCPs (i.e., two, three, or four electrodes for applying high voltage) is simulated. The results show that two MCPs’ multiplication structure with the four-electrode assembly method is an appropriate choice.