Cosmic Rays

          

          Cosmic rays were discovered by Victor Hess and Werner Kolhörster in the beginning of the twentieth century. In 1912, Victor Hess found the ionization rate increased approximately fourfold over the rate at ground level (carried Wulf electrometers to an altitude of 5300 metres in a free balloon flight). In 1913-1914, Werner Kolhörster confirmed Victor Hess earlier results by measuring the increased ionization enthalpy rate at an altitude of 9 km. Hess received the Nobel Prize in Physics in 1936 for his discovery.

           Cosmic rays are highly penetrating radiations which are continually entering the Earth’s atmosphere (at nearly the speed of light) in all directions from outer space.

They originate from the Sun, from outside of the solar system and from distant galaxies. Particles from outside of our solar system are called Galactic Cosmic rays (GCRs). Data obtained from the Fermi Space Telescope (2013) shows that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Cosmic rays consist of high energy charged particles. Most of these particles have energy of the order of 15 GeV.

Origin of cosmic rays:

          Several theories have been put forward regarding the origin of primary cosmic rays, and the mechanism by which they acquire enormous energies.

(i) Explosion theory:

          Lemaitre, Regener and others have suggested that once upon a time the whole mass of the universe was concentrated in a single nucleus. About three billion years ago, this universe exploded, forming the galaxies which are still running away from each other. During the explosion, a fantastically great amount of radiation was formed. Protons and other nuclei were shot out in all directions with all energies and in sufficient numbers. The cosmic rays are simply the debris, the dust of the explosion. This view does not explain the presence of the heavy nuclei in primary rays. Such a tremendous explosion should have broken up the heavy nuclei into their constituent particles, viz., protons and neutrons.

(ii) Origin from Sun:

          One of the views is that the sun may be the source of at least some of the cosmic rays. According to it, at the times of solar activity violent eruptions occur and ionized gases shoot out from the sun. Thus some of the protons in the sun acquire high energies and thrown out into interplanetary space. The points in favour of this assumption are that the cosmic ray intensity increases during solar flares and varies slightly with the rotation of the sun. But since the cosmic ray intensity remains almost uniform at all hours of the day and night, sun cannot be thought to be responsible for the majority of the primary cosmic ray particles. It may be the source of the small fraction of the low energy primaries.

(iii) Origin from cosmic ray stars:

          Another view is that the cosmic rays come from the so called cosmic ray stars which are more active than the sun. Our galaxy has about 10¹¹ stars including double stars, variable stars, novae and supernovae. All these may be the possible origins of cosmic rays.

          The current view of the origin of cosmic rays is that the sun emits low energy cosmic rays while high energy cosmic rays are emitted by cosmic rays stars within our galaxy. Regarding the tremendous energies of cosmic rays, most scientists believe that protons acquire acceleration in interstellar magnetic fields.

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 consist mainly of positively charged atomic nuclei with Z upto about 40. About 90% of the primaries are protons, 9% helium nuclei and the remaining heavy nuclei. The energies of primary cosmic rays range from 1 MeV to 10¹⁴ MeV.

Secondary cosmic rays:

          When primary cosmic rays interact with the nuclei of atmosphere gases, secondary cosmic rays are produced. Below an altitude of 20 km, all cosmic radiation is secondary. When primary cosmic rays enter the Earth’s atmosphere, they collide with molecules of gas. It produces mostly π-mesons (positive, negative and neutral) and some hyperons. 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 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.