At the end of July, a huge cave was completed in Japan to house the world's largest neutrino detector, Hyper-Kamiokande. The National Centre for Nuclear Research (NCBJ) is also actively involved in the project, both in terms of scientific work and the manufacture of measurement equipment.

Hyper-Kamiokande is an emerging observatory and neutrino experiment led by the University of Tokyo and the KEK research centre in collaboration with institutes from around the world. The project aims, among other things, to investigate the possibility of proton decay, measure symmetry-breaking effects between neutrinos and antineutrinos, and observe neutrinos from supernovae. The experiment is also expected to contribute to a better understanding of the Grand Unification Theory and the evolutionary history of the Universe.

The excavation of the cavern that will house the Hyper-Kamiokande experiment took almost three years. The huge cylinder-shaped space topped by a dome is almost 100 metres high and 70 metres in diameter. This makes it one of the largest man-made areas excavated in bedrock in the world. In the next stages of the experiment's construction, the cave will be transformed into a huge water tank. Specially designed photodetectors and associated electronics will then be installed in it. More than 600 people from 22 countries are currently involved in the Hyper-Kamiokande experiment. A strong group of Polish scientists from nine research centres, including the National Centre for Nuclear Research, is also involved. Polish scientists are responsible, among other things, for significant contributions to the construction of the experiment's detection system and the preparation of tools for the reconstruction and analysis of neutrino interactions.

A dedicated linear electron accelerator with an extensive beam guidance system will also be built at the National Centre for Nuclear Research. The purpose of the installation will be to deliver calibration beams of precisely selected energy to the detector. Full control over the beam, its energy and position are currently the most important technological challenges facing NCBJ scientists.

Hyper-Kamiokande will be a detector that uses the Cherenkov radiation phenomenon to record the charged particles produced by neutrinos and measure their energy. The experiment under construction will be more than 8 times larger (260 kilotons of water mass) than the Super-Kamiokande detector currently operating in Japan. The advantage of this type of experiment is its simple operating principle: the area in which neutrinos are detected is a large volume filled with ultra-pure water, surrounded by light detectors (photomultipliers). Neutrinos passing through the detector can interact with electrons in the water, resulting in charged particles moving faster than the speed of light in the water. This leads to the emission of cone-shaped Cherenkov radiation. This phenomenon is the optical equivalent of the shock wave created when breaking the speed of sound in air. This radiation, with its characteristic blue colour, is recorded in the form of circles by photomultipliers covering the walls of the tank. From this, it is possible to reconstruct the parameters of the particle that caused the effect, as well as the neutrino itself.

The difficulty in building Hyper-Kamiokande is the enormous size of the cylindrical tank, 73 m high and 69 m in diameter, which required a suitably large cave to be hollowed out of the bedrock. The whole structure has to be placed deep underground (about 600 m), for shielding the detector from cosmic radiation. This size and location present new experimental and engineering challenges, as it will be the largest facility of its kind in the world.

Details of the completion of the Hyper-Kamiokande cave excavation are available on the website: https://www-sk.icrr.u-tokyo.ac.jp/en/news/detail/1006

Project website: https://www-sk.icrr.u-tokyo.ac.jp/en/hk/