Muons are the most numerous
energetic charged particles at sea level. A charged particle cannot avoid
losing energy by ionization. As it passes through matter the charged particle
interacts with the electric fields and typically knocks loose some of the
loosely bound outer electrons. A muon interacts very little with matter except
by ionization. Because of this, muons can travel large distances and commonly
reach the ground. However, they lose energy proportional to the amount of
matter they pass. This is proportional to the density (g/cm3) times
the path length (cm). This "interaction length" has units of grams
per square centimeter. (see “Interaction Depth” later)
Muons lose energy at a fairly
constant rate of about 2 MeV per g/cm2. Since the vertical depth of
the atmosphere is about 1000 g/cm2, muons will lose about 2 GeV to
ionization before reaching the ground. The mean energy of muons at sea level is
still 4 GeV. Therefore the mean energy at creation is probably about 6 GeV.
The atmosphere is so tenuous at
higher altitudes that even at 15,000 m it is still only 175 g/cm2 deep.
Typically, it is about here that most muons are generated. Muons arrive at sea
level with an average flux of about 1 muon per square centimeter per minute.
This is about half of the typical total natural radiation background.
Muons (and other particles) are
generated within a cone-shaped shower, with all particles staying within about
1 degree of the primary particle's path.
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