Air Showers are the phenomena which allow us to view and make observations into cosmic rays. These are simply the showers of elementary particles produced by high energy particles hitting the atmosphere at velocities that are significant fractions of the speed of light
A Cosmic Ray particle can have a general maximum kinetic energy on the order of 500 EeV, which is a meaningless number when you consider it in electron volts because few people realize how incredibly big an Exa-something is. This energy can be converted to about 5 Joules, for a single particle on the order of a proton, alpha particle or sometimes an iron nucleus. For further clarification, you might do well to consider that a thrown baseball generally has kinetic energy of 10 J. While the vast majority of CR particles are not in this range, the likelihood of an air shower produced by such particles is much higher than for particles with lower energy. There is an exception to this rule which was about 10 times as powerful as this high end figure.
Note: It is an interesting prospect to think about such energies in the context of space travel, where the mercy of our atmosphere is not present. Cosmic rays have been known to leave spider cracks in the windows of space habitats and space suits.
When all of this energy comes in contact with particles in the atmosphere the result is tiny, short-lived fission events that release large numbers of high energy elementary particles. The act of hitting the atmosphere with that much energy releases a cascade of X-Rays, as well as every other elementary or theoretical particle imaginable. The closest humanity has come to reproducing these sort of interactions and explosions of particles is within particle accelerators where only now are large hadrons being accelerated to energies in the TeV range. The energies involved here are still a million times greater than what we can produce. As a result the study of cosmic ray air showers is of particular importance to particle physics.
When the particle hits some nucleus in the atmosphere, it splits and in splitting releases a number of positive and neutral pions as well as secondary cosmic rays, which are the now very energetic nuclei produced in the collision. Some of the pions manage to hit other particles and cause furthers splitting. These particles continue to lose energy but are eventually visible at various prepared observation arrays set up with X-Ray cameras, Geiger-tubes and underground photomultiplier pools, looking for that Cherenkov glow.
So far the record held for most energetic particle is a whopping 4.8 ZeV, or 48 J. If you can imagine almost all the energy of a man with baseball bat hitting the nucleus of an oxygen atom, you can see that this is a huge number. This event was called the Oh-my-God particle, named amusingly for the Higgs Boson, which is sometimes referred to as the 'God Particle.' This was detected in 1991 by the Fly's Eye, a multifaceted photomultiplier tube detection unit.
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