Water Cerenkov detectors are similar to the scintillator detectors, a description of scintillation detectors is given below, except instead of a scintillator it is filled with pure, clear water; also known as heavy water.

Heavy water is chemically the same as regular (light) water, but with the two hydrogen atoms (as in H₂ O) replaced with deuterium atoms ( hence the symbol D₂O). Deuterium is an isotope of hydrogen; it has one extra neutron. Thus the deutrium atom consists of one proton and one neutron in the atomic nucleus and one orbiting electron. It is the extra neutron that makes heavy water "heavy", about 10% heavier actually.

When cosmic rays pass through the water they emit faint flashes of blue       light known as Cerenkov radiation.

When light travels through any material such as glass or air, it slows down slightly to a speed less than the speed of light. Some high energy particles, such as cosmic rays can travel faster than this. When theat happens the particles emit fain flashes of blue light known as Cerenkov radiation. This light is vaguely analogous to the sonic boom produced by an aircraft that travels faster than the speed of sound.

It is important to remember that the cosmic rays are not travelling faster than "c". They           are just travelling faster than light does after it has been slowed down by passing through             a material.

                                                                                                                                                           The sides of the water tank are lined with reflective material and some of     the cerenkov radiation is reflected onto a photomultiplier which produces     an electronic signal. The size of the signal can be used to find out how many cosmic rays passed through the detector.

The figure above is a photomultliplier. A photomultiplier, as its name suggests, multiplies the small flash of light into a large electrical signal that can be measured. From the size of the electronic signal we can tell how many particles passed through the scintillator. The scintillator and the photomultiplier are housed in a dark box so that the only light detected is caused by cosmic rays.

To read more about Cerenkov Detectors use the following link

Andre Gaudin's Detector Page

Scintillation Detectors:

The scintillation detector is made up of a special piece of plastic called a `scintillator'. When fast moving, charged particles, such as cosmic rays pass through the scintillator they excite the atoms in the plastic by giving them some energy (the cosmic ray then slows down a little). The excited atoms then lose this energy by emitting  photons of light. The light is detected by a sensitive piece of equipment called a "photomultiplier".

Andrea T. Hughes - March 2004