Performance of the MCP-PMT for the Belle II TOP counter in a magnetic field

The time of propagation (TOP) counter is a novel particle identification device for the Belle II experiment, utilizing the ring imaging Cherenkov technique. It is composed of a 2.7 m long quartz bar and 32 micro channel plate photomultiplier tubes (MCP-PMTs) attached on the end of the bar. The MCP-PMT has a typical quantum efficiency of 28%, a photoelectron collection efficiency of about 55%, and a transit time spread (TTS) less than 50 ps for single photon detection. Since the TOP counters are installed in the 1.5 T magnetic field of the Belle II detector, the MCP-PMTs are required to keep those characteristics with gain of more than 5×105 in the magnetic field. Therefore, we investigated gain, TTS, and single photon detection efficiency of the MCP-PMTs in the magnetic field and found that they satisfy the requirements. We are starting complete inspections for more than 500 MCP-PMTs, and have inspected up to 87 MCP-PMTs.