Cosmic-ray air showers
The earth is hit by elementary particles and atomic nuclei of very
large energies. Most of them are protons (hydrogen nuclei) and
all sorts of nuclei up to uranium (although anything heavier than
nickel is very, very rare). Those are usually meant when talking
about cosmic rays. Other energetic particles in the cosmos
are mainly electrons and positrons, as well as gamma-rays and neutrinos.
Interactions and particle production
The cosmic rays will hardly ever hit the ground but will collide
(interact) with a nucleus of the air, usually several ten kilometers
high. In such collisions, many new particles are usually created
and the colliding nuclei evaporate to a large extent.
Most of the new particles are pi-mesons (pions). Neutral pions very
quickly decay, usually into two gamma-rays. Charged pions also
decay but after a longer time. Therefore, some of the pions may
collide with yet another nucleus of the air before decaying,
which would be into a muon and a neutrino.
The fragments of the incoming nucleus also interact again, also
producing new particles.
The gamma-rays from the neutral pions may also create new particles,
an electron and a positron, by the pair-creation process.
Electrons and positrons in turn may produce more gamma-rays by
the bremsstrahlung mechanism.
The number of particles starts to increase rapidly as this shower
or cascade of particles moves downwards in the atmosphere. On their
way and in each interaction the particles loose energy, however,
and eventually will not be able to create new particles. After some
point, the shower maximum, more particles are stopped than created
and the number of shower particles declines. Only a small fraction
of the particles usually comes down to the ground. How many actually
come down depends on the energy and type of the incident cosmic ray
and the ground altitude (sea or mountain level). Actual numbers are
subject to large fluctuations.
In fact, from most cosmic rays nothing comes down at all.
Because the earth is hit by so many cosmic rays, an area of
the size of a hand is still hit by about one particle per second.
These secondary cosmic rays constitute about one third of the
When a primary cosmic ray produces many secondary particles, we call this
an air shower. When many thousand (sometimes millions or even
billions) of particles arrive at ground level, perhaps on a mountain,
this is called an extensive air shower (EAS). Most of these particles
will arrive within some hundred meters from the axis of motion of
the original particle, now the shower axis. But some particles can be
found even kilometers away. Along the axis, most particles can
be found in a kind of disk only a few meters thick and moving almost
at the speed of light. This disk is slightly bent, with particles
far from the axis coming later. The spread or thickness of the
disk also increases with distance from the axis.
Extensive air showers with many particles arriving on the ground
can be detected with different kinds
of particle detectors.
In the air the particles may also emit light by two different
processes: Cherenkov light almost along the shower axis and
fluorescence light in all directions.
Other introductory material found on the net (HTML format):
Cosmic Rays by Richard Mewaldt
Cosmic Rays by James Schombert
Further reading found on the net (Postscript or PDF format):
Particle Data Group: review of cosmic rays
Introduction to high energy cosmic ray physics
Cosmic Ray Spectrum and Composition: Ground Observations
This page was written by Konrad Bernlöhr.