Water Cherenkov Tanks
To record the particles created in cosmic-ray and gamma-ray air showers, the HAWC detector uses the water Cherenkov method. In this technique, the detector is used to sample air-shower particles at ground level by recording the Cherenkov light produced when the particles pass through pass through tanks full of purified water.
The water Cherenkov detectors in HAWC are corrugated steel tanks 4 meters high and 7.3 meters in diameter (see figure at left). Each tank contains a watertight bladder and four photomultiplier tubes (PMTs) which are sensitive at ultraviolet wavelengths. Three of the four photomultipliers are 8-inch hemispherical Hamamatsu PMTs previously used in the Milagro experiment. These three PMTs are placed on the bottom of the tank looking upward, and spaced several meters from the center of the tank. The fourth PMT, positioned at the bottom of the tank in the center, is a high quantum-efficiency 10-inch PMT designed to increase the efficiency of the observatory to low-energy showers (<1 TeV).
The figure at right shows a simulation of a single air-shower muon penetrating one of the tanks. The particle propagation code GEANT4 was used to produce the Cherenkov photons created by the muon and trace them through the water inside the tank.
Cherenkov production is relatively efficient in water due to its high index of refraction. The Cherenkov light is emitted into a forward cone that surrounds the direction of motion of the charged particle. The opening angle of the cone depends on the index of refraction of the medium. For example, in air, where the index of refraction is nair = 1.0003 the opening angle of Cherenkov radiation is about 1°. In water where nwater = 1.33 the opening angle of Cherenkov radiation is 41°. Because the Cherenkov cone in water is so large, nearly every charged particle that enters the tank should be observed by at least one of the four PMTs.
Note that the water Cherenkov tanks can be used to detect charged particles and gamma-rays inside the shower. The water is dense (relative to air), and so a gamma-ray produces an e+e- (electron/positron) pair once it enters the tank. These charged particles then emit Cherenkov radiation as they speed through the water.