NATURE OF COSMIC RAYS | STRUCTURE OF SHOWER FORMATION

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).