Particle Physics Group,Department of Physics, Graduate School of Science, Kobe University
Super-Kamiokande (SK) is a water Cherenkov-type detector installed 1,000 meters underground at the Kamioka Observatory, part of the Institute for Cosmic Ray Research, University of Tokyo, located in Hida City, Gifu Prefecture. It is designed for research in astrophysics and particle physics. The detector consists of 50,000 tons of ultra-pure water and approximately 11,000 photomultiplier tubes, each 50 cm in diameter. It is the largest underground water Cherenkov detector in the world.
Its main observation targets include cosmic neutrinos (from the atmosphere, the Sun, supernovae, etc.), artificial neutrinos (from the T2K experiment, described later), and the search for nucleon decay.
The Super-Kamiokande experiment is conducted by an international collaboration of about 230 researchers. Since observations began in 1996, the detector has operated continuously, 24 hours a day, 365 days a year, except during periods of maintenance and upgrades.
Our laboratory is particularly engaged in several key research initiatives: the study of low-energy neutrinos using the SK detector; the development of the SK-Gd project (EGADS), which aims to detect neutrinos emitted from past supernova explosions with high sensitivity; and the construction and research of the Hyper-Kamiokande (HK) experiment.
The Super-Kamiokande detector is a cylindrical structure made of stainless steel. To distinguish charged particles entering from outside, the detector features a dual-layer design consisting of an "inner tank" and an "outer tank." The outer tank is 41.4 meters tall and 39.3 meters in diameter, and it contains approximately 50,000 tons of ultra-pure water infused with gadolinium (Gd).
The inner tank holds about 32,000 tons of water and is lined with approximately 11,000 photomultiplier tubes (PMTs), each 50 cm (20 inches) in diameter. PMTs are devices that convert light into electrical signals, and the 20-inch PMTs used in this experiment are the largest of their kind in the world, originally developed for the predecessor experiment, Kamiokande.
To maximize the detection of light generated within the inner tank, about 40% of the inner tank’s wall surface is covered with these 20-inch PMTs.
When a charged particle has high energy and travels faster than the speed of light in a medium, it emits a type of light known as Cherenkov radiation, which is released in a cone-shaped pattern in the forward direction of the particle's motion. In the SK detector, this faint Cherenkov light is captured by photomultiplier tubes (PMTs) mounted on the inner surface of the detector. By analyzing the timing, intensity, and pattern of the Cherenkov light, researchers can determine the time of occurrence, energy, and direction of the incoming charged particle.
Neutrinos themselves carry no electric charge and therefore do not directly produce Cherenkov light. However, when an incoming neutrino interacts with the ultra-pure water in the inner tank of the SK detector, it can generate high-speed charged particles. By detecting the Cherenkov light emitted by these secondary particles, researchers can study the properties of the original neutrino.
