The Super-Kamiokande detector consists of a cylindrical stainless steel tank, 39.3m in diameter and 41.4m in height, and photo sensors called photo-multiplier-tubes (PMTs) installed to the detector wall. The detector is filled with 50 ktons of pure water.
The PMT support structure divides the tank into two distinct, optically isolated volumes, the inner detector (ID) and the outer detector (OD). The inwarded PMTs are installed to the ID wall and the outwarded PMTs to the OD wall.
The tank must be kept close because we must take data continuously. The photographs inside of the tank were taken during the construction and reconstruction period.
Photo-Multiplier-Tubes (Inner detector)
The inward-facing 11,129 photo-sensors, called photo-multiplier-tubes (PMTs) are instrumented in the inner tank. The Cherenkov light emitted by charged particles running in water is detected by these sensors. The energy and direction of the charged particle is calculated by the information of the detected charge and timing. This PMT is developed with Hamamatsu-photonics. The diameter of the photosensitive area is 20 inch (about 50cm) and is the largest in the world.
2In November 2001, an apparent cascade of implosions triggered by a single PMT implosion destroyed over half of the PMTs in water.
After 2002, in order to avoid chain reactions of implosions even if a single PMT implosion is happen, all of inner PMTs are covered with acrylic and fiberglass shields.
The inner PMTs are instrumented at intervals of 70cm and cover about 40% of the detector wall. The rest of the surface is covered with black polyethylene terephthalate sheet which optically separates the tank to the inner part and the outer part.
Photo-Multiplier-Tubes (Outer detector)
Super-Kamiokande detector is optically separated into two concentric cylindrical regions by PMT support structures and pairs of opaque sheets. In the outer region, 1,885 outward-facing PMTs are instrumented. The PMTs of the outer detector(OD) are smaller than those of the inner detectors and the diameter is 8 inch (about 20cm). The OD PMT is attached to a 60cm x 60cm wavelength shifter plate in order to collect photons efficiently. The walls are covered with reflective white sheet.
The main purpose of the outer detector is to distinguish the neutrino events from the cosmic ray muon events. The cosmic ray muons are background sources of the neutrino observation. Because the neutrinos are electrically neutral, the Cherenkov light is emitted when the neutrinos scatter with the water. On the other hand, because the cosmic ray muon have an electric charge, the Cherenkov light is immediately emitted when the cosmic ray muon enters the detector. Therefore, in case of cosmic ray muons the OD PMTs almost have hits, while in case of neutrinos, the OD PMTs have few hits. This difference enables to roughly distinguish between neutrinos and cosmic ray muon events.
In the four huts on the water tank, the electronics system which read out the information of the signal sent from the photo-multiplier-tubes and the high voltage power supplies are located.
A high voltage of about 2000 volts is supplied to each PMTs. The signals from about 13,000 PMTs are sent to the electronics system in four electronics huts via 70m cables.
This electronics system digitalizes the analog signals of PMTs and obtains the charge information and timing information of detected photon.
This electronics system digitizes the analogue information about the amount of detected charge and the hit timing from PMTs. In September 2008, the data acquisition system of Super-Kamiokande has been updated for the first time since the Super-Kamiokande started the observation. New system has recorded all the hits of each PMT which hits about 4,500 a second for analysis. The stored data size in a day becomes as much as 500GB.
Water Purification System
The Super-Kamiokande detector is filled with 50,000 tons of ultra-pure water. It is very important to keep the water clean to improve the detection precision.
The Super-Kamiokande uses the clean groundwater. The further removal of small dusts, ions, bacteria and radon can reduce the scatter of Cherenkov light and background noise from radon decay products in the water. The water in the tank is continuously reprocessed in the cycle system. The typical number of particles of size greater than 0.1 micrometer in the water is reduced to 100 particles/cc after purification.
Top of Tank
Electronics huts and several calibration equipment are located on the tank of the Super-Kamiokande. The roof of the cavity is a dome shape in order to disperse the weight of the 1000m mountain above the tank.
The rock in the mine contains the radioactive material (radon) whose decay products become one of the background sources to neutrino observations. To avoid contamination of the radioactive material emitted from the rock, the ceiling and the wall are sealed by the polyethylene Mineguard . Furthermore, a "radon-free" air is piped into the tank top from the outside to further hold back contaminated mine air.
Super-Kamiokande continues to take data for 24 hours a day 365 days a year. The researchers always keep watching the status of the detector and checking the data quality on three shifts. In this control room, two researchers do various checks during the first shift (8:30am-16:30pm). During the other shifts, researchers continue to check from the laboratory which is located in 15 minutes by car from the entrance of the mine.