Speaker
Description
Hyper-Kamiokande, the successor to Super-Kamiokande, is a next-generation water Cherenkov detector scheduled to begin operation in 2028. It aims to measure neutrino oscillation parameters, including the CP-violating phase and the mass ordering, with significantly higher precision than Super-Kamiokande. This improvement is enabled by its fiducial volume, which is 8.4 times larger, leading to statistical uncertainties smaller than the systematic uncertainties. Systematic uncertainties are also expected to be reduced by the newly developed 50-cm photomultiplier tube (PMT), R12860, which achieves approximately twice the photon detection efficiency and a twofold improvement in charge and timing resolutions.
To further reduce systematic uncertainties, this study investigates the PMT response uniformity with respect to the light incident position on the photosensitive area. Here, the PMT response refers to the detection timing, charge, and efficiency. By precisely modeling these response parameters as functions of the light incident position, the systematic uncertainties arising from the variations in the vertex direction can be suppressed. To refine the model, we investigate differences among individual PMTs and the impact of varying conditions, including high voltage, ambient magnetic fields, and light incident angles.
To carry out this investigation, we established an experimental setup for measuring the PMT response uniformity. First, a robotic arm was used to control the position and angle of an optical fiber tip from which light was emitted toward the PMT, enabling precise and efficient adjustments. Second, the measurements were conducted in the center of Helmholtz coils to provide a controlled uniform magnetic field. Finally, the incident light was split, and intensity fluctuations caused by arm movement were monitored using a silicon photomultiplier (SiPM), a photodetector insensitive to magnetic fields. This setup enabled measurements of detection timing, charge, and efficiency as functions of the incident light position and angle, magnetic field, high voltage, and individual PMTs.
As a result, non-uniformities in the response were observed, appearing to reflect the geometry of the photocathode and dynodes. The magnetic field exhibited non-uniform effects. For example, under a 100 mG magnetic field, the detection timing shifted by about ±2 ns depending on the position. On the other hand, variations in high voltage had a relatively uniform influence over the entire photosensitive area. Regardless of the high voltage, the gain varied by less than 5%, and the detection efficiency varied by less than 10%, except near the edge of the PMT. All parameters were fitted using a model function that incorporates the geometry of the PMT.
In conclusion, this study clarified the response of the new 50-cm PMT, including its dependence on various parameters, and developed a non-uniform response model. This model can be applied to improve event reconstruction and detector simulation, thereby contributing to enhanced precision in measuring neutrino oscillation parameters.