Performance evaluation in different environments of the MCP-PMT for the TOP counter in the Belle II experiment

At the Belle II experiment, a Time-of-Propagation (TOP) counter is used for charged particle identification in the barrel region. The TOP counter employs micro-channel-plate photomultiplier tubes (MCP-PMTs), which have excellent time resolution. One of the issues for the TOP counter is the photocathode lifetime of the MCP-PMT. During the Belle II experiment, quantum efficiency (QE) was found to decrease more rapidly than expected from pre-installation lifetime tests. In these proceedings, we investigate the effect of environmental conditions on the QE degradation. Lifetime measurements at different temperatures showed no significant effect on the QE degradation. Afterpulse and lifetime measurements of the MCP-PMTs with and without a magnetic field revealed that the magnetic field affects the QE degradation and indicated a possible contribution from ions.