The picture below shows the inside of one of the KASCADE huts; they are the little white bungalows in the background picture on the top of this page.

In the bottom of these huts, there is a slab made from a certain plastic, covered in black tape to keep ambient light out. When energetic particles traverse this scintillator, they produce a tiny flash of Čerenkov light. This flash is picked up by photomultipliers, the metal cones on top of the scintillator, and translated into an electric signal which is fed through meters and meters of cable into a central computing facility. If several particles traverse the scintillator et the same time, the signal picked up by the photomultipliers is also higher.

By combining the signals of several of these detectors over a large area, one can estimate the total number of particles in an air shower, from which one can estimate the energy of the original, primary particle impinging on the Earth.