Cosmic Radiation Cosmic radiation is a high energy particle radiation from outer space. According to our present knowledge, sources of such radiation are our sun (solar wind), other stars, but also Supernova explosions, cosmic jets, pulsars, and black holes. The achieved particle energies can be much larger than what can possibly be achieved in modern laboratory particle accelerators. In particular at very large particle energies, the sources and hence the galactic acceleration mechanisms of the radiation and not known and hence focus of modern research. The primary cosmic radiation consists of 87% protons (hydrogen nuclei), 12% alpha particle (helium nuclei), and 1% heavier atomic nuclei. The electrically charged components of the radiation is guided via the earth’s magnetic field to the pole regions, while the neutral component travels undisturbed. At an altitude of approximately 13 km, the primary radiation hits the oter atmosphere. Consequently, the protons and alpha particle break up producing mostly pions (bound states of up- and down-quarks or anti-quarks). The neutral pions decay to photons and create electromagnetic showers. The electrically charged pions decay into electrons and electron neutrinos or muons and muon neutrinos. In that decay, electrons are suppressed with respect to muons by a factor 10000. This is a consequence of the properties of the weak interaction. All in all, muons decay to (99,98770±0,00004) % into muons. The primary cosmic radiation also interacts directly with the atmosphere, producing hadrons which in turn react with the atmosphere and decay quickly. In summary, the secondary cosmic radiation consists of 80% muons, almost 20% electrons and almost 1% hadrons. As muons have a lifetime of about 2.2 microseconds, following the laws of classical mechanics they could only travel 660 m and hence not reach the ground of earth. Due to their high velocity and their large Lorentz-boost factor (βɣ>16), their time dilation is so large that they can easily travel distances of 11 km or more and hence reach the earth’s ground level. The average flux of cosmis muons at sea level altitude is approximately one muon per cm² and minute. The exact rate depends on the considered muon direction and the minimum muon energy required for their detection.