In the Ultra High Energy regime, direct detection using satellites or balloons is not feasible because of the extremely low rates. A detector covering hundreds of square miles is needed. Fortunately the Earth's atmosphere itself can be used as a detection medium, producing "extensive air showers" from primary Ultra High Energy cosmic rays as they enter.
An extensive air shower begins with the collision of a primary cosmic ray with a nucleus (usually nitrogen) near the top of the atmosphere. This first collision typically produces more than a thousand secondary particles. These particles are emitted very close to the direction of the original particle. They still retain an enormous amount of energy, so this jet of new particles repeats the collision process in a cascade which can grow to billions of particles.
In each collision, some of the kinetic energy of the incoming particle is converted into mass energy (new particles) and the remaining kinetic energy is then distributed over the secondary particles in the shower. This process of redistribution of energy continues until the average energy of the shower particles is insufficient to produce more particles in subsequent collisions. This point of the extensive air shower development is called "shower maximum"; it contains the greatest number of particles.
An extensive air shower develops and propagates at nearly the speed of light. This takes place over a few millionths of a second. Most primary cosmic ray collisions occur at high altitude, above 15 miles elevation (80,000 feet.) Only a few primary cosmic ray particles ever survive to even the highest mountain altitudes. All that is detectable at ground level are secondary particles which arrive at sea level at a rate of one through an area the size of your head each second.
The mirrors in the "fluorescence" telescopes gather ultraviolet light and focus it onto sensitive cameras consisting of a grid of Photomultiplier Tube pixels which convert the light into an electric signal.
The scintillation process is also a component of the surface detector array. Each surface detector is composed of two panels of acrylic plastic which has been infused with a scintillating material. Energy deposited by charged particles passing through the material is converted into luminescent light. This light is gathered by fiberoptic cables and concentrated onto photomultiplier tubes