Sep 12 – 16, 2022
University of Edinburgh
Europe/London timezone
RICH2022 in Edinburgh is dedicated to the memory of Jacques Séguinot (1932-2020)

MCP-PMT quantum efficiency monitoring and operation status of the TOP counter at the Belle II experiment

Sep 16, 2022, 9:00 AM
25m
University of Edinburgh

University of Edinburgh

presentation Cherenkov light imaging in particle and nuclear physics experiments Photon detection techniques for Cherenkov counters

Speaker

R. Okubo (Nagoya University)

Description

The BelleⅡ experiment is a high luminosity electron and positron collider experiment at SuperKEKB in Japan. In this experiment, we aim to measure B-decay precisely and search for effects from New Physics. We started physics data taking with the whole detector system in March 2019. The Time-of-propagation (TOP) counter is a detector for particle identification in the barrel region of the BelleⅡ detector. It consists of a quartz radiator and high timing resolution photodetector, and it can identify $K^{\pm}$ and $\pi^{\pm}$ from the arrival time and hit position of Cherenkov light.
Micro-Channel-Plate Photomultiplier(MCP-PMT) is the photodetector for the TOP counter; it measures photon timing with a resolution of 30 ps; it gives excellent particle identification performance. One of the issues for the TOP counter is the lifetime of the photocathode of MCP-PMTs. The quantum efficiency (QE) will decrease by the accumulated output charge of MCP-PMTs due to the outgassing from MCPs. We have worked to improve the lifetime of the photocathode and installed three types, Conventional type, Atomic Layer Deposition (ALD) type, and Life-extended ALD type, in the TOP counter. The lifetime of the conventional MCP-PMT is 1.1 $\mathrm{C/cm^2}$ on average, and we are planning to replace it with the Life-extended ALD type that has the longest lifetime during the long shutdown that starts in 2022 summer.
We have developed monitoring tools for MCP-PMTs and measured the gain, output charge, and QE of MCP-PMTs during physics data taking. First, we measured the gain, and it changed about 10$\%$ during physics data taking due to the characteristics of ALD that applied to MCP for a longer lifetime. To make the output charge of MCP-PMTs smaller, we are operating with relatively low gain. Due to this, threshold efficiency will decrease during physics data taking. We increased the high voltage for MCP-PMTs during physics data taking, considering gain decrease and output charge to keep efficiency. Second, we measured the output charge of MCP-PMTs. ( Figure1 ) These are small enough compared to their lifetime, and we expect that there is almost no QE degradation in all MCP-PMTs. Finally, we measured the QE degradation considering the threshold efficiency drop during the physics data taking. ( Figure2 ) As a result, we found a more considerable QE degradation than expected in 5$\%$ of MCP-PMTs. We think this degradation is a problem of the MCP-PMT production, noise from the read-out system, etc.
We will present the MCP-PMT status and the operation status of the TOP counter at the BelleⅡ experiment.

Primary author

R. Okubo (Nagoya University)

Presentation materials