Cosmic
Rays are continually entering the earth’s atmosphere in all direction from the
outer space. From large number of experiments, it is firmly established that
the interaction with the earth’s atmosphere clearly distinguished cosmic rays
into two categories, which is primary and secondary cosmic rays.
Primary
Cosmic Rays
The
cosmic rays which are just entering our earth’s atmosphere from outer space are
called primary cosmic rays. Primary cosmic rays have their origin somewhere out
in space. They travel with speeds almost as great as the speed of light and can
be deflected by planetary body or inter galactic magnetic fields (Strong et al, 2002). The composition of cosmic
rays entering the earth’s atmosphere is fairly well known from balloon
experiments and it is found that these primary cosmic rays consist of mainly of
fast protons. Primary cosmic rays consist of positively charged atomic nuclei
with atomic mass Z of about 40. About 90% of the primaries are proton, 9%
helium nuclei and 1% heavy nuclei. The radiation reaching the earth is almost
completely isotropic (Clay and Davison, 1997).
As soon as the primary cosmic
rays enter the earth’s atmosphere multiple collisions readily take place with
atmospheric atoms producing a large number of secondary particles in showers. Thus
when a primary proton strikes an oxygen or nitrogen nucleus a nuclear cascade
results. The energy spectrum of the primary cosmic rays ranges from 109 eV
to about 1019 eV and can be expressed as
dN/dE=
K (E + m0c2)-𝛾
where
N is the number of nuclei with a kinetic energy per nucleon > E (in GeV) and
m0c2 is the nucleonic rest energy. K and 𝛾 are constants for a given cosmic ray component (Kakani and
Shubhra, 2008).
2.2.2 Secondary
Cosmic Rays
When primary cosmic rays interact
with the nuclei of atmospheric gases, secondary cosmic rays are produced. They
are produced as a result of inelastic collisions of primary rays with the
nuclei of the oxygen and nitrogen atoms of the air in the upper layers of the
atmosphere. Below an altitude of 20 kilometers (Km), all cosmic radiations are
secondary (Pachoff, 2009). On entering the atmosphere, the primary cosmic rays
collide with air nuclei and produce mostly 𝜋-mesons
(positive, negative and neutral) and some hyperons (secondary cosmic rays). The
𝜋-mesons so produced carry
sufficient energy and decay into lighter particles: 𝜇-mesons, electrons, positrons, neutrinos and
photons. All such particles constitute the secondary cosmic rays.
At high energy of primary particles (greater
than 5 x 109 eV), their collisions with the atoms of air lead, as a
rule to the initiation of electron-nuclear showers. The result of interaction
between the particles and the nuclei of atoms of the air is the disrupting of
the latter into separate nucleons or lager fragments, and the formation of
unstable particles (𝜋˖ and 𝜋0 mesons). The subsequent decays
𝜋+ ⟶ 𝜇+ + v (neutrino)
𝜋- ⟶ 𝜇- + v
𝜋0 ⟶ 𝛾 + 𝛾
𝛾 ⟶ e+ +
e
lead
to the formation of a soft
electron-photon component of the shower. This component is then intensively
multiplied due to the consecutive (cascade) formation of new e+- e-
pairs and to bremsstrahlung of new gamma quanta by the particles (electron
cascade process) as shown in Fig 2.1 below, the totality of nuclear high energy
interactions brings about extensive air
showers. At energies of the primary particles over 1010 eV,
these showers may contain millions of particles with the transverse dimensions
of the shower exceeding 1 km2 (Grieder, 2001).
At sea level the secondary
cosmic rays contain nearly 70% 𝜇˗mesons, 29%
electron-positron pairs and 1% heavy particles. The mesons in the secondary
cosmic rays constitute the hard component and the electrons, positrons and
photons constitute the soft component (Kakani et al, 2008).
