The rays which receive on earth from outer space are called cosmic rays. It has been found that extremely penetrating radiations consisting of charged and uncharged particles and photons are reaching the earth. These radiations are called Cosmic Radiations. The sources of these radiations lie beyond the sun or the other stars of our galaxy.
The radiations reaching the top of the atmosphere of the earth consist of protons of high energy and a few nuclei such as helium, lithium, and Beryllium. These radiations which are reaching the top of the atmosphere of the earth from remote sources and which consist of mostly protons are called Primary Cosmic Rays.
Where do cosmic rays come from?
During the year 1990, it was found by the C.T.R Wilson, Gietel and Elster that the divergence of the leaves of a charged electroscope slowly decreased in spite of best insulations showing that the charge of the leaves could be due to the presence of ions around the leave. The ions which carried opposite to that of the leaves could slowly neutralize the charge of the leaves.
These ions, which possibly neutralized the charge of the electroscope, might have been produced by some kind of rays. These rays were called cosmic rays because their source is found to be outside of the earth and the atmosphere of the earth because there was found to be very little change in the intensity of these rays with the distance from the earth because of their absorption in the earth’s atmosphere.
There was also found to be only a small change in the intensity of these rays during solar and lunar eclipses. This showed that the source of cosmic rays was somewhere beyond our galaxy.
In 1910, Glockel carried his electroscope up to a height of about 3 miles and was surprised to observe that the rays of the discharge were increased. In 1911, Hess discovered that the rate of the discharge increased with the height, and so he suggested that some unknown penetrating radiation was entering the earth’s atmosphere from space. For this reason, Hess is given the credit for the discovery of this mysterious radiation, called cosmic rays, and he was granted the Nobel Prize in physics for the year 1936.
Milikan studied the intensity of cosmic rays at different places. His observations indicated that the intensity was constant at all latitude. This led him to think in 1930 that the primary cosmic rays, which enter our atmosphere, are high-energy gamma rays. This view was found to be wrong as explained below
- Altitude Effect
The measurement of the ionization in the air at different heights with the help of airplane and balloon flights shows that the intensity of the cosmic rays increases with height reaches a maximum, and then decreases. The height, at which cosmic ray intensity becomes maximum depends upon the latitude. For example, the maximum is found at a height of 15 miles at an altitude of 51°N, and the maximum intensity is 170 times as great as the sea level. The variation in the intensity of the cosmic rays with altitude is called the altitude effect.
- Latitude
In 1913, Clay measured the cosmic ray intensity at various latitudes during his voyage from Amsterdam to Batavia. His observation indicated that the cosmic ray intensity (at sea level) remains constant at latitudes greater than 45°N or 45° S, but appreciably decreases at the magnetic equator i.e., The intensity has a maximum value at the equator. This is called Latitude Effect.
Nature of Cosmic rays
The altitude and the latitude effects can be explained by supposing that the primary cosmic rays, which come from the outer space, are almost entirely composed of high energy positively charged particles which are mostly protons. Before reaching the surface of the earth, the particles have to pass through the earth’s magnetic field. The positively charged particles moving towards a pole travel parallel to the direction of the earth’s magnetic field and so experience no force. Hence all the particles moving toward a pole can reach the surface of the earth.
The positively charged particles moving normally to the surface of the earth of the magnetic equator travel perpendicular to the earth’s magnetic field of the equator and so these particles experience and a force perpendicular to the direction of the motion, which makes them move in a circular arc. If the velocity of a particle is sufficiently slow, it will be a turn back and so it will not reach the earth at the equator. It is obvious that some of the particles moving toward the equator will not be able to reach the earth, and so the cosmic ray intensity is expected to be less at the equator than at the poles. This explained the latitude effect.
At a height of about 15 miles, the density of the air is appreciable and so several collisions take place between primary cosmic rays and atomic nuclei. Such collisions produce a number of nuclear fragments and highly penetrating gamma –rays., which are called “Secondary Cosmic Rays.” As a single primary cosmic ray particle can produce fresh secondary ray by further collision with the atomic nuclei, the intensity of the cosmic rays (primary and secondary) come nearer the earth’s surface, the density of the air rapidly increase and the so the ray distance of the cosmic rays from the surface of the earth goes on decreasing. This explains the altitude effect.
Primary cosmic rays are high-energy particles. It found that some primary cosmic rays particles possess energies as high as 10° Mev. How and where such high-energy particles are produced in the universe is still a mystery.
Origin of Primary Cosmic Rays
T he primary cosmic rays which consist of protons and some heavier nuclei are found to approach the earth from all directions.
The energy of primary cosmic rays has been found to be in the range of 109 to 1017 e.g. No definite answer has been found regarding the origin of the cosmic rays. However, the following suggestion has been made:
- It has been suggested that interstellar space beyond our galaxy contains photonic matter. The turbulent motion of this charged matter produces a very strong magnetic field which accelerates the protons toward the earth with very strong energy.
- Fermi has suggested the origin of the primary cosmic rays in our own galaxy. The diffuse hydrogenous matter present between stars is ionized by the photons emitted by the stars. Since our galaxy is rotating, therefore, the rotation of this diffuse protonic matter produces a strong magnetic field that accelerates the protons and impart them with very high energies. This theory fails to explain the presence of heavier nuclei in the primary cosmic rays.
- It has also been proposed that the origin of the primary cosmic rays lies within the solar system. The cosmic rays have been supposed to originate from the sun or from places near the sun. the heavy nuclei and protons might be found near the sun and might be accelerated by the magnetic fields of the solar system. But there has been a very small decrease in the intensity of the cosmic rays during the solar eclipse. It proves that the origin of the cosmic rays is beyond the solar system.
Cosmic Rays Shower
Rossi, in 1932, with his coincidence counter technique, found that cosmic rays arrive in groups that appear to have been produced by the same cause are “shower”. The showers consist of electrons, positrons, and photons.
The shower particles appear to leave the point of origin with the cone 40°.
A high-energy proton of the primary cosmic rays may collide with an atom of the earth’s atmosphere and may knock out an electron in the source of the cosmic ray shower.
The high-energy electrons lose their energy by colliding with some atoms and emitting photons. The photons produce electrons and positrons through the process of Pair Production.
Earth particle of the pair collides with some atoms and emits, in turn, electromagnetic radiation. The photons so produced again produce electrons and positrons till the energy of each electron and positron is insufficient to produce photons.
Also, a meson may decay to produce a high-energy electron to initiate a shower. The shower stared by a high-energy electron.