Discovery of cosmic rays:
It has been seen that a well-insulated gold leaf electroscope slowly loses its charge, even when apparently no ionising agents are. present. At first, it was assumed that the loss of charge of the electroscope was due to the ionising radiations coming from radioactive substances present in the earth. But experiments made by Millikan on the loss of charge of an electroscope in lakes, about a mile below the water level revealed that there was no ·decrease in the rate of discharge.
On the, other hand further experiments indicated that the rate of loss of charge increases as we go higher above the earth. From these observations, on the variation of cosmic ray intensity with altitude and depth, it was concluded that a penetrating radiation was incident on the earth from all directions and that it was coming from interstellar space. This radiation was, given the name cosmic rays.
What are Cosmic Rays?
Cosmic rays consist of high energy atomic nuclei, mainly. protons reaching our earth from outer space as well as secondary radiation produced by these nuclei in the earth’s atmosphere. Most of these particles have energy of the order of 15 GeV (1 GeV = 1 billion electron volts) but occasionally some particles are found to posses energies as high as 1011 GeV which is a billion times as large as that attained with the help of the largest existing particle accelerator.
Primary cosmic rays and its constituents.
The primary cosmic rays are those which are initially incident upon the outer boundaries of the atmosphere. It is now well established that the primary cosmic rays are very high energy protons and to a lesser extent heavier nuclei which rain upon the earth from all the directions in the outer space.
The primary cosmic radiation approaching the earth’s atmosphere contains about 92% protons, 7% alpha particles and I% still heavier nuclei of almost all the atoms from Li to Ni (A< 60). The relative abundance of the various nuclides present in the primary cosmic rays is almost the same as the relative abundance of the elements in the earth’s crust as well as in the stars.
Secondary cosmic rays and its constituents.
The primary cosmic ray particles, mostly protons, some alpha-particles and a few heavier nuclei interact with the nuclei of atmospheric gases and give rise to secondary cosmic rays, consisting of a hard component and a soft component. Below an altitude of 20 km all cosmic radiation is secondary.
When an O ‘or N nucleus in the atmosphere is struck by a’ high energy cosmic ray proton it can gain a large amount of energy and then disintegrate into afas.t proton, a fast neutron and charged as well as neutral pie-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 emulsion, they present the appearance of a star. At sea level about 70% of the secondary cosmic rays are mesons, 9% are electron-positron pairs while 1% are heavy particles. The mesons in the secondary cosmic rays constitute the hard component ana electrons; positrons and photons the soft component. These cosmic rays when enters the earths atmosphere leads to the occurrence of cosmic ray showers.
Altitude effect.
The nature and origin of cosmic rays can be known to some-extent by studying the, variation of the intensify of cosmic rays with altitude-and depth.
The ionisation produced by cosmic rays at higher and higher altitudes reaching practically upto the top of the atmosphere has been studied with the help of cosmic ray stations at the top of the mountains, aeroplanes, manned balloons balloons fitted with self-recording instruments rockets and satellites.
It is seen that the intensity increases slowly upto a height of about 8 km and thereafter rapidly upto 20 Km. At. a height beyond 24 km the intensity starts decreasing slightly. All the four curves at magnetic latitude 30° N, 38° N, 51° N and 60° N, however, show that on nearing the magnetic equator the cosmic ray intensity decreases at high altitudes as well as at sea level.
At an altitude of about 25 km the secondary cosmic ray particles outnumber the primary particles by almost fifteen times. But as these particles travel downwards, they progressively lose energy by breaking the nuclei an ionising the air until they are in thermal equilibrium with air molecules and cease to exist as charged particles. However, high speed secondaries and a few primaries do reach sea level and some of them still have sufficient energy to penetrate several thousand meters of earth and water.
Cosmic ray intensity at the equator.
The decrease in cosmic ray intensity at the earth’s magnetic equator is now explained as being due to earth’s magnetic field. The earth’s magnetic field is weak but it extends far out of earth’s atmosphere and can effect the path of incoming
Charged particles even at large distances from the earth the paths of all charged particles crossing the earth’s magnetic field are bent by a force that is perpendicular to the direction of the field. The amount of deflection depends upon the momentum and charge on the particles. All particles with energies less than 104 MeV are so strongly deflected by the earth’s field at the equator that they are unable to enter the earth’s atmosphere.
In fact at low energies, the particles which do not travel parallel to the magnetic lines of forces spirals around these lines as the particles approach the earth, the field strength increases, the radii of their spiral paths become smaller and smaller till ultimately they are so sharply bent that they get reflected back and spiral round the lines of force again approaching the earth 9n the other side.
Such charged particles travelling to and fro constitute the Van Allen belts, which surround the earth except at the magnetic poles. The outer belt is constituted by the low energy particles like protons and electrons from the sun and the inner belt is due to more energetic ones from outer space.
The particles approaching the earth in the direction of its magnetic axis are not deflected by magnetic field. Hence primary cosmic particles of any energy can enter the earth’s atmosphere near the magnetic poles. But particles with only the highest of energies can get down to the earth’s atmosphere near the magnetic equator. This explains the latitude effect.
