The Super-Kamiokande experiment mainly observes neutrinos by using a huge water tank with about 13,000 PMTs.
50,000 ton water target
Neutrinos do not have the electric charge and go through matter, so it is very difficult to detect them. However, a neutrino occasionally interacts with matter and a charged particle is generated. The charged particle can be detected. The 50,000 tons of water as a big target can increase the number of interaction between neutrinos and nucleons or electrons.
13,000 eyes watch Cherenkov light
When the generated charged particle in water has higher velocity than the speed of light in water, the Cherenkov light is emitted. This phenomenon is similar to that the diagonal wave is made on the water surface, when a duck moves at a higher velocity than the speed of the water surface wave.
The Cherenkov light is emitted in a cone shape to the direction of a charged particle. The photo-multiplier tubes (PMTs) of the wall of the tank detect this Cherenkov light. The PMTs have information of the quantity of the detected light and the timing of the detection. The energy, direction, interaction point and type of the charged particle are decided by the information from the PMTs.
This picture is the event display of a muon neutrino detected by the Super-Kamiokande. The colored points indicate the quantity of the detected light by each PMT. The Cherenkov ring emitted by a muon is displayed.
Mountain as Umbrella
The primary cosmic ray (mainly protons) is continuously poured on the earth. When the cosmic ray interacts with the atmosphere of the earth, muons, electrons and neutrinos (called secondary cosmic ray) are generated. Many muons lose their energy and stop in the ground. On the other hand, neutrinos do not stop because they rarely interact with matter.
The mountain above the detector plays the role of an umbrella and protects from cosmic ray muons which become the background of neutrino observation. The 1000m rock overburden reduces the cosmic ray muons to 1/100,000 of the ground surface.