The primary cosmic ray particles mostly protons, some alpha-particles and a few heavier nuclei interact with the nuclei of atmospheric gases giving rise to secondary cosmic rays. For example, when an O or N nucleus is struck by a high energy cosmic ray proton, it can gain a large amount of energy and then disintegrate into a fast proton, a fast neutron and charged as well as neutral π-mesons or pions. Other particles such as anti-protons, anti-neutrons and heavy particles known as hyperons are also produced. All these particles leave the site of collision with high energies. If these particles are observed by their tracks in photographic emulsions, they present the appearance of a star.
The fast protons and neutrons interact further with atmospheric nuclei and produce a nucleonic shower of low energy neutrons and protons that reach the earth’s surface or sea level.
The charged pions, π+ and π- (which have a rest mass of 273 times the mass of the electron), being short lived (half life 2 x 10–8 sec) decay to give high energy μ-mesons or muons of the same sign and a light weight neutral particle called the neutrino. Recent determination of the rest mass of μ-meson gives its value as 207 times the rest mass of the electron. The μ-meson interacts weakly with the nuclei. This is the reason why it possesses a very penetrating power in matter and can reach the sea level after penetrating considerable distances.
Thus, at sea level, the hard component of secondary cosmic rays consists mainly of strongly penetrating muons and some low energy nucleons (protons and neutrons).
The charged muons μ+ and μ-, as such exhibit only a small tendency to interact with nuclei and most of them decay to an electron or positron and two neutrinos as the case may be. The decay scheme for the μ-mesons is as under
How cosmic ray showers are produced?
The decay of the pions π+ and π- into the corresponding muon and a neutrino and further decay of muon μ+ or μ- into a positron or electron and a neutrino and anti-neutrino. The neutral pions π0 decay to two y-ray photons. Depending upon the energy of μ0-meson, y-ray photons can have energies sometimes over 100 MeV. Such high energy y-ray photon interacts with matter and each of them gives rise to an electron positron pair.
Thus, we find that from a single proton we get a whole series of tiny particles. This explains the presence of cosmic ray showers in the upper regions of the atmosphere.
The resulting-electrons and positrons produced by the decay of π0 -pions in the upper regions of the atmosphere while passing very close to the nucleus of an atom of the atmospheric gas interact with their nuclear field and generate a y -ray photon each, which in their turn behave in the same manner i.e., each first produces a photon and then an electron positron pair and so on till the energy falls below a certain critical value. The number of particles, therefore, goes on multiplying and we get an electron photon shower.
The phenomenon of showers was observed by Blackett and Occhialini who studied the cosmic rays with the help of a cloud chamber and their satisfactory explanation on the above lines was given by H. Bhaba and Hietler in 1937.
Thus, at low altitudes the soft component of secondary cosmic rays mostly consists of electrons, positrons and Gamma(y)-rays. The hard component accounts for nearly 75% and the soft component about 25% of the cosmic rays at sea level.
