Soon after 1900 it was shown by scientists that the air in an ionisation chamber, which was completely protected against penetration of α , β and γ rays, by surrounding it with thick lead walls, was still conductive; and it was thought that the inns causing this conductivity were produced by some rays coming from an unknown source. In 1911, Hess and Kolhoster, by placing ionization chambers in balloons and sending then to great heights established the existence of penetrating radiations coming from above. These radiations are now called Cosmic Rays
Further investigations on these rays were begun by Millikan in 1925 by means of sounding balloons carrying a recording electroscope, thermometer and barometer; and also by lowering measuring instruments deep into lakes. It was found that the intensity of the penetrating rays (i.e., Cosmic Rays) increases with the height, reaching a maximum at a height, equivalent to 9 meter of water, above earth, which shows that most of the rays must actually be produced there , and much of the cosmic radiation observed at lower level is a secondary radiation, produced by the impact of the original radiation with air molecules.
It has also been noticed that the intensity of cosmic rays is greater for higher magnetic latitudes than for lower latitudes. This seems to be due to the fact that the magnetic field of the earth deflects the cosmic ray particles, although some of then are still able to reach the earth , while others do not. Also it has been observed that the number of particles coming from the west is greater than from the east , and the effect is known as The East West Effect.
The cosmic radiations are of two types:
Those which penetrate about 10 cm of lead are called Hard Cosmic Radiations and those which are absorbed within this thickness of lead are soft.
The great penetrating power of the cosmic rays was at first believed to show that the rays must be photons of the same nature as γ rays. In 1926, Bothe and Kolhoster showed that the cosmic rays consist largely of electrically charged particles and the primary or original cosmic rays consist of α particles, protons and electrons.
In 1932 Anderson used Wilson cloud chamber and powerful magnetic field to bend the path of the cosmic ray particles. He discovered that there was present a new type of particle, which has the same mass as an electron but has positive charge equal to that of an electron. This particle is called positron, the name negatron is sometime given to the ordinary electron to distinguish it from a positron as stated.
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A photon can be broken into an electron and a positron. This process is called Pair Production.
Cosmic rays have been investigated by means of Wilson cloud chambers and tracks which are due to secondary radiations have been obtained. There tracks show that they originate from the same point and many goog-ring particles emanate from the same point. Such a group of particles form a shower, a Cosmic Shower. A theoretical explanation of their showers has been given by Dr. Homi Bhabha & Heitler.
It is assumed that the shower producing raw is a fast moving electron. When it passes close to the nucleus of an atom at A (see image), a photon radiated which consequently passes close to the nucleus of another atom as at B and produces a +ve & a -ve electron(?), i.e., a positron and a negatron(electron). Meanwhile the original electron produces another photon at C, which in turn produces another electron pair at D and so on, thus producing fresh chains, constituting what is known as the Cascade Process. This process continues so long as the energy of the electron and photon is greater than a critical value & performes showering. Note that A, B, C, D…are nuclei. And a high energy γ ray / photon passing close to the nucleus of an atom is annihilated and electrons/positrons are produced.