Difference between Gamma-Rays and Cosmic Rays - by Albert Lim (2000)
Gamma-rays differ considerably from cosmic rays both in characteristics as well as
in origin and behaviour. As mentioned above, Gamma-rays are photons constituting the
most energetic form of electromagnetic radiation. They are photons with the highest frequency
and energy of any electromagnetic radiation and they have characteristic wavelengths
from about 10^-2 nanometers or shorter. Cosmic rays on the other hand, have nothing to
do with the electromagnetic radiation. Cosmic rays are in fact high energy charged particles
consisting mainly protons and atomic nuclei (98%) and electrons (about 2%). Some cosmic
rays have energies a billion times greater than those that can be achieved in particle
accelerators. In general, most cosmic rays reaching the earth are of lower energy. The
key to the high energies possessed by cosmic rays are in their high velocities - a very
small mass travelling at very high velocity still posses very high kinetic energy.
Fig 2 : A cosmic ray magnesium nucleus, entering from top left. It
collides with a bromine nucleus in photographic emulsion and both
nucleus shatter into a multitude of fragments.The cosmic track of the
magnesium nucleus from top left to the colliding point is 0.18mm long
It is interesting to note that a small fraction constituting 0.1% of cosmic rays are
in fact photons in the form of gamma-rays. These cosmic gamma-rays are produced in collisions
of high energy particles. Because cosmic rays carry electrical charges, their trajectories
are constantly affected by interstellar magnetic fields making it impossible for astronomers
to confirm the directions from which they originate. Gamma-ray photons on the other hand
are neutral and they therefore travel in straight lines and are not affected by magnetic
fields. Unlike cosmic rays, astronomers can therefore confirm directions from which gamma-rays
are coming from. While gamma-rays are often accompanied by alpha and beta radiations,
cosmic rays are not. Alpha and Beta radiation do not actually involve the emission of
electromagnetic radiation but are instead electrically charged subatomic particles. The
extraterrestrial origin of cosmic rays was determined in 1911 by Victor Hess and so named
by Robert Millikan in 1925. Gamma-rays were first studied by Ernest Rutherford when he
looked at the radiation of thorium and discovered an extremely penetrating radiation
- much more so than the alpha and beta radiation he had discovered earlier in 1897-1898.
The origins of cosmic rays are still relatively uncertain, as are the processes by which
they are accelerated. Origins of gamma-rays on the other hand are better known in recent
years as are their various originating processes such as particle-particle collision,
matter-antimatter annihilation, radioactivity decay and acceleration of charged particles.
