Specifications of the Detector
The Hyper-Kamiokande detector will be constructed 600 m underground at the Kamioka Mine in Hida City, Gifu Prefecture, for the purpose of observing neutrinos, proton decay, and other physics.
It is a type of detector called a “water Cherenkov detector.” The experiment will be conducted in a huge cylindrical water tank, 68 meters in diameter and 71 meters deep, filled with extremely clear “ultrapure water.” It is the world’s largest underground water tank.
Two-layer water Cherenkov detector
The Hyper-Kamiokande detector is a ‘water Cherenkov detector’ and is filled with 260,000 metric tons of ultrapure water. The detector fiducial volume of Hyper-K is 10 times larger than that of Super-Kamiokande. The Hyper-K detector will be world’s largest underground water tank.
Each of the Hyper-Kamiokande detector is divided into an “Inner Detector” and an “Outer Detector” (see Fig. 2), that are optically separated from each other.
The Inner Detector is the main detector, with 40,000 max. 50cm-diameter ultrasensitive photo-sensors installed on its walls. The ultrasensitive photo-sensors allow us to measure a very weak Cherenkov light, generated from nucleon-decays and neutrino interactions, precisely. Through using these ultrasensitive photo-sensors, Hyper-Kamiokande is able to discriminate between muons and electrons with more than 99% accuracy. Such a great particle identification capability is one of the features of the Hyper-Kamiokande detector.
The Outer Detector surrounds the Inner Detector and is equipped with 10,000 max. 8cm-diameter ultrasensitive photo-sensors. A primary function of the Outer Detector is to reject the incident cosmic-ray muons that make up part of the background in the measurement of nucleon-decays and neutrinos. The rejection efficiency of cosmic-ray muons reaches more than 99.9% with the Hyper-Kamiokande detector.
The Hyper-Kamiokande detector is the world’s largest precision measuring instrument for nucleon-decay and neutrino studies. Hyper-Kamiokande is able to investigate multiple topics within physics, with the aim of discovering new phenomena by obtaining physically significant results at the highest precision for each of the topics of interest to the physics community. Please visit the “Physics” section for further details of the physics topics targeted by Hyper-Kamiokande.
We have been developing the world’s largest photosensors, which exhibit a twice higher photodetection efficiency than that of the Super-Kamiokande photosensors. These new photosensors are able to measure light intensity and its detection time with a much higher precision.
Photosensors in water Cherenkov detectors
Large water Cherenkov detectors such as Super-Kamiokande and Hyper-Kamiokande require large, highly sensitive optical sensors to measure the extremely faint light (Cherenkov light) generated by neutrino interactions and proton decay.
In Super-Kamiokande, a photomultiplier tube with a diameter of 50 cm and the world’s largest photosensitive area, is used as the light sensor. The light received by the photomultiplier tube is converted into electrons, which are accelerated in the vacuum by high voltage. Inside the photomultiplier tube, the number of electrons increases as an avalanche each time they strike metal plates called a dynode. By counting these faint light particles as electrical signals, Super-Kamiokande observes the Cherenkov light information using about 10,000 photosensors.
Each photosensor measures “when” and “how much” light has arrived. This detection performance has a significant impact on the overall observational performance of the water Cherenkov detector. For example, a more accurate measurement of the time light detected will allow more precise estimates of the point of origin of neutrino interactions or proton decay. Furthermore, the more detection light, the more accurately we can estimate the energy of the particles produced in the interactions. These performance improvements also make it easier to eliminate noise events, known as background.
Two new types of photosensor
If we can improve the observational performance of the detector with better-performing photosensors, we will be able to study target physics more deeply with higher sensitivity. The Hyper-Kamiokande uses 40,000 photosensors in an inner tank with a depth of 71 m, which is greater than the 41 m depth of the Super-Kamiokande, so the new photosensors must have high performance and be able to withstand high water pressure.
We are considering using two types of photosensors in Hyper-Kamiokande.
The first is a new large-aperture, a high-sensitivity photomultiplier tube (PMT) that has approximately doubled the basic performance of the PMTs used in Super-Kamiokande in terms of light detection efficiency, light intensity measurement accuracy, and time measurement accuracy. The strength has also been improved to withstand approximately twice as much water pressure. In addition, the background radioactive material content has been reduced to half. We made several major improvements to achieve these. For example, we improved the manufacturing process of the photocathode, and the shape of the metal plate that initially receives the electrons was changed, resulting in high sensitivity to detect faint light. In addition, the dynode structure, which amplifies the number of electrons, has been significantly improved to measure the timing and the light intensity.
Mass production of the new photomultiplier tubes began in 2020. The delivered sensors have been stored until the day to be mounted on the wall, after confirmation and performance measurement for safe long-term use. We plan six years of manufacturing until the experiment starts in 2027.
The other is a “compound eye” photosensor. Multiple 8cm diameter photomultiplier tubes with high detection time accuracy are bundled and sealed together to enable detailed imaging as if to increase the resolution of a camera. The project is being completed through international cooperation between Japan and other countries such as Italy, Canada, and Poland.
By using two types of photosensors together, detection accuracy is expected to be improved.