Kamioka ObservatoryICRR, The University of Tokyo

Excavation of the gigantic underground cavern that will house the Hyper-Kamiokande experiment began in November 2022 in Kamioka-cho, Hida City, Gifu Prefecture Japan. Excavating what will be the world’s largest artificial underground cavern, is the climax of Hyper-Kamoikande’s construction. The Hyper-Kamiokande experiment is the next international research project in a line of experiments in Kamioka, the Kamiokande experiment and the Super-Kamiokande, both of which led to Nobel Prizes in physics. It aims to elucidate the evolution of the universe and the fundamental theories of elementary particles by using the world’s largest underground detector to observe neutrinos and search for proton decay. Collaborating researchers visit the site daily and discuss the design and planning of both the cavern and detector, with the goal of starting the experiment in 2027.

The challenge of the third Kamiokande

Following both the first generation experiment Kamiokande, which spurred the rapid development of neutrino research around the world, and the second generation experiment Super-Kamiokande, which discovered properties of neutrinos one after another, the third-generation experiment Hyper-Kamiokande will further open the future of neutrino research with its improved experimental capabilities and precision neutrino observation techniques built upon the success of its predecessors.

The Hyper-Kamiokande experiment aims to elucidate the fundamental theory of the origin of matter and elementary particles in the universe with precise measurements of neutrinos  and unparalleled searches for proton decay. Hyper-Kamiokande was officially launched in February 2020 as an international research project and now over 500 researchers from about 20 countries are participating. Construction of the Hyper-Kamiokande detector is currently underway and the experiment plans to start taking data in 2027.

The Hyper-Kamiokande detector will be installed 600 m underground beneath a mountain in Kamioka-town, Hida City, Gifu Prefecture Japan. It is a cylindrical water tank with a total mass of 260,000 tonnes, of which the effective mass for observations is 190,000 tonnes, approximately ten times the volume of Super-Kamiokande. The walls of the tank will be fitted with some 40,000 ultra-sensitive optical sensors, which are twice as sensitive as those in Super-Kamiokande. Thanks to the larger size and improved performance of these sensors, Hyper-Kamiokande is expected to further elucidate the properties of neutrinos, to observe rare events such as proton decay, and to study astronomical phenomena by observing neutrinos emerging from celestial objects.

Professor Masato Shiozawa of the Institute for Cosmic Ray Research (ICRR) at The University of Tokyo, the co-spokesperson of the Hyper-Kamiokande, emphasizes the importance of the project saying, “Hyper-Kamiokande will be a project that will challenge the mysteries of elementary particles and the universe through observations of the enigmatic elementary particle known as the neutrino and through searches for proton decay. Together with our international collaborators, we hope to contribute to the world’s knowledge of particle physics and astronomy by realizing the full potential of this revolutionary project.”

 

 

Keeping the excavation work on schedule with the highest safety standards

The main underground cavity in which the Hyper-Kamiokande detector will be installed  is 69 m diameter, 73 m tall cylinder with a 21 m high domed support. It will be the largest artificial underground cavity in the world. Various geological investigations have been carried out since 2003 in preparation for the unprecedented challenge of excavating this giant cavity. Researchers have carefully planned the design of the detector and excavation of the cavern through repeated site visits and consultations with experts from various fields.

 

 

In FY2020, by using a newly constructed 96 m long survey tunnel and a 725 m long bore hole,  an extensive survey was carried out to study the state of the rock at the planned detector location in detail. In addition, around the tunnel entrance a yard, an electric distribution system as well as a water supply and drainage system were built. Construction of the access tunnel to reach the main underground cavity began in May 2021.

 

 

Excavation of the 1.9 km tunnel took about nine months, and excavation of the approach tunnels beyond it began in February 2022. The approach tunnels attach to the detector’s main cavity in three branches – at the bottom, middle and top of the main cavity. In June 2022 the tunnel to the top reached the center of the cavity dome. Excavation of the sub-cavities around the main cavity followed.

Project Associate Professor Yoichi Asaoka (ICRR, The University of Tokyo), the deputy leader of the far detector facility group, said, ” Despite the various constraints imposed by COVID-19, we were able to complete the approach tunnels on schedule. There was no water inflow,　which is often a problem in tunnel construction, and by making full use of long-hole blasting and other excavation techniques, we were able to dig through the tunnel in a short period of time. It only takes a moment to create a delay but the only way to make up for any delay is with daily effort. In cooperation with our contractors, excavation work is carried out with the utmost regard for safety first while prioritizing the demands of the site. Moreover, we are very grateful for the cooperation of local residents and Hida City. In order to meet their expectations, we hope to contribute to the invigoration of Kamioka Town and Hida City by starting the experiment as planned and achieving long-term stable operation.”

 

 

Starting the excavation of the the world’s largest artificial underground cavern

Finally, excavation of the giant underground cavern needed to house Hyper-Kamiokande began in November 2022. This is the most crucial phase of the detector construction. The dome section above the main cavern was excavated in a pattern like that of a snail’s shell.  Anchors were embedded in the cavern ceiling to help expand the space while ensuring the stability of the bedrock.

When completed, the dome section will have a diameter of 69 m. It must withstand the pressure of the 600 m of mountain above and maintain a stable space for the detector. Work to expand the dome is carried out carefully, but quickly, with safety as the top priority. The key to securing the large underground space is the “Hida gneiss”, one of Japan’s most solid rock formations, which was formed under great pressure during ancient times. In total the mountain is supported by more than 600 anchors. The stability of the bedrock is confirmed by comparing daily measurement data with simulations. Accordingly, the length and spacing of the anchors as well as the excavation sequence are carefully planned before each excavation step. In two years the excavation of the cavities will be completed. Afterwards construction of the water tank will begin.

Associate Professor Shoei Nakayama (ICRR, The University of Tokyo), the leader of the far detector facility group, expressed his aspirations, “We are excavating a huge underground cavern with the largest size in human history and the excavation of the dome is a particularly difficult task that is expected to present a variety of challenges.  Everyone involved will work together to overcome the difficulties and thereby give momentum to the construction work that will follow.”

 

 

International cooperation

The Hyper-Kamiokande project is an international collaborative scientific project hosted by Japan. Japan’s share of the project is the excavation of the underground cavity for the Hyper-Kamiokande detector, the water tank and light sensor support structure, half of the inner water tank light sensors, the main part of the water circulation system, the first-stage data analysis system, the J-PARC accelerator and beamline upgrades, and facilities for the so-called near detector at J-PARC. Other participating countries are responsible for the shock-proof covers for the light sensors, the other half of the inner tank light sensors, the outer tank light sensors, the data readout system, the detector calibration system and the near detectors themselves . International participants are currently securing funding and preparations are underway for the production of detector components throughout the collaboration.

Professor Francesca Di Lodovico (King’s College London), the co-spokesperson of Hyper-Kamiokande, comments, “The unique capabilities of Hyper-Kamiokande, and excellent track record of the Kamiokande series of experiments have attracted international interest. Groups from many countries are providing their expertise to ensure Hyper-Kamiokande achieves its full potential and from 2027 produces high-quality data that will be essential for scientists worldwide for many decades to come.“ The Hyper-Kamiokande collaboration aims begin operations as planned in 2027 and is working to realize this potential through the collective wisdom and cooperation of researchers from all over the world.