Kamioka ObservatoryICRR, The University of Tokyo

The Hyper-Kamiokande detector will feature a cylindrical water tank measuring 68 meters in diameter and 72 meters in height. Frames for mounting its photosensors are being installed inside the tank. The design includes inward-facing photosensors with a diameter of 50 cm and outward-facing sensors with a diameter of 8 cm. Preparations for these sensors are progressing steadily.

There will be two types of inward-facing light sensors. One is a newly developed photosensor that has approximately double the performance of the sensors used in Super-Kamiokande in terms of light detection efficiency, observed light intensity, timing accuracy, and durability. Production of this new sensor began in 2020, and over 10,000 units have been manufactured by November 2024. Completed sensors undergo regular inspections by researchers, who check their appearance and quality and conduct performance measurements.

A test called the “100-sensor measurement” involves placing 100 photosensors in a darkroom, powering them on, and checking their electrical signal output. In a completely dark environment, the photosensors are expected to operate stably. However, during long-term measurements, rare cases of instability and malfunctions can occur. This testing ensures that such issues are virtually eliminated before installation, paving the way for reliable, long-term operation of the Hyper-Kamiokande detector.

Check out the time-lapse video of the “100-sensor measurement” here.

The other type is a “compound-eye” photosensor, known as the mPMT module.　This design combines 19 smaller 8 cm photosensors into a 50 cm diameter configuration, enabling detailed imaging capabilities. The development of these sensors is being carried out by research groups from Italy, Canada, Poland, the Czech Republic, and Mexico.

By utilizing both types of light sensors together, the detection accuracy of the Hyper-Kamiokande detector is expected to improve significantly.

Meanwhile, the 8 cm diameter photosensors for the outer detector are being developed primarily by groups from the UK, South Korea, and Australia, with support from Japanese researchers.

Prototype photosensors under consideration were tested in a darkroom to evaluate their performance. Measurements were performed to test their response to a light source, including the signal variation over time and noise levels. Based on these results, the final specifications for the detector will be determined.