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.