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, such as the CP phase and mass ordering, with much higher precision benefiting from a fiducial volume 8.4 times larger than that of Super-Kamiokande. The newly developed 50-cm photomultiplier tube (PMT), R12860, manufactured by Hamamatsu Photonics K.K., provides two times higher resolution for both charge and timing measurements, as well as twice the detection efficiency. However, possible non-uniform response across its large photosensitive area may serve as a source of systematic uncertainty. To reduce and quantify this uncertainty, it is necessary to evaluate the PMT response uniformity under realistic conditions, including different high-voltage (HV) settings and ambient magnetic fields.
To evaluate the PMT uniformity, the PMT response was measured at more than 1,000 photon incident positions. At each position, various PMT properties were measured, including the single photoelectron charge distribution (from which gain was obtained), cathode transit time difference (CTTD), transit time spread (TTS), and relative detection efficiency. The photon incident position was controlled by a robotic arm, with the light source kept at a constant distance and oriented perpendicularly to the PMT surface so that the laser beam spot size and reflectivity remained unchanged. These uniformity measurements were repeated under different HV settings and magnetic fields, the latter controlled by a Helmholtz coil.
In a 0 mG environment, variations in response depending on the photon incident position were observed, appearing to correspond to the shape of the dynode. The influence of high-voltage differences had a larger impact on the response than the dependence on the photon incident position. However, the effect on uniformity could be compensated by a constant factor that did not depend on the position. Regardless of the HV, the gain ratio varied by less than 4%, and the detection efficiency ratio by less than 10%, except at the very edge of the PMT. A magnetic field was found to affect the uniformity response, introducing an asymmetric dependence on the azimuthal angle of the PMT position. For instance, in a 100 mG environment, the CTTD shifted by about ±2 ns depending on the direction of the magnetic field and the photon incident position. These uniformity results were parametrized in a model function of the photon incident position, HV, and magnetic field, which can be incorporated into event reconstruction and simulation programs, and applied to correct the PMT response for variations in the operational conditions in Hyper-Kamiokande.
In conclusion, the uniformity study of the 50-cm PMTs has clarified the key dependencies more comprehensively than previous studies, and this has enabled us to establish a PMT response model. This model will be utilized to suppress systematic uncertainties, thereby contributing to the precise determination of oscillation parameters.
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