Performance of the MCP-PMTs of the TOP Counter in the First Beam Operation of the Belle II Experiment

Abstract

We developed a square-shaped micro-channel-plate photomultiplier tube (MCP-PMT), and successfully produced and installed 512 MCP-PMTs into the TOP counter of the Belle II experiment in 2016. The TOP counter is the first-ever detector that is equipped with such a large number of MCP-PMTs. All of them have a time resolution better than 50 ps for single photon detection and a peak quantum e ffi ciency of 29.3% on average at a wavelength around 360 nm. Those excellent time resolution and e ffi ciency are essential for the TOP counter to reconstruct the Cherenkov image for particle iden-tification. The MCP-PMTs were operated in success for the first beam data taking of the Belle II experiment in 2018. The number of detected photons per track from the beam collisions was evaluated, and it was found to be larger than expected, which is yet to be investigated.