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-- NigelWatson - 07 Sep 2010

Cosmic Rays

Cosmic rays are high energy charged particles that originate in outer space, (mainly from supernova explosions but also from stars see "where do cosmic rays come from" to read more) travel at the speed of light and strike the Earth from all directions.

They strike the Earth's surface at the rate of about 1 cosmic ray every square centimetre every minute. Perhaps a more meaningful way to look at this is that during an average nights sleep a person will have on average of a million cosmic rays traveling through their body!

Cosmic Rays typically have energies ranging between 106 - 1020 eV (around 10-13 - 10-10 Joules). Although in these terms this does not sound a very large amount of energy, the energy is very concentrated; the particles are extremely small (typically 10-29kg and of the order 10-18m in radius, or 0.000000000000000001 metres in radius) it can be appreciated that this is a large energy for each of these minute particles to have.

Cosmic rays are very penetrating; so much so that they have also been detected under the ground, such as in the London Underground system and down deep mines. We can get Primary and Secondary Cosmic Rays, to be discuessed later on.

Where do they Come From?

All the light and heat we get on Earth comes from the sun, so it may be a resonable assumption that cosmic rays also originate from it. However this is wrong, two pieces of experimantal data support this conclusion.

The first one is rather intuitive as we all know it's light in the day and dark at night as well as warmer in the day than at night. These fluctuations in heat and light levels, all coming from the sun, would suggest that the intensity of cosmic rays would vary throughout the day as well. However it has been found that the levels remain constant throughout the day thus dispelling the idea that they come from the sun.

The second piece of evidence is that the sun would be unable to give out particles with the high energy levels that cosmic rays are known to have.

We now believe that cosmic rays originate in our own galaxy for a few stars, rarely, and mainly from SuperNova? -when a star reaches the end of its lifecycle and explodes. When the explosion occurs it releases many high energy atomic nuclei in the form of mainly Hydrogen and Helium nuclei, it's these that are the source of primary cosmic rays.

For a more indepth review of the evidence for this conclusion please Click here

Who discovered them?

In 1912 Austrian-American physicist and hot air enthusiast Victor Hess found that his electroscope discharged more as he gained altitude in a balloon. He concluded that there must be radiation entering our atmosphere from outer space, which he dubbed cosmic rays. In 1936 he was awarded the Nobel Prize for his discovery.

Uses In Science

Cosmic Rays are very useful in many areas of science e.g.:

  • Due to their high energy they can be used to smash atoms so we can learn more about their internal stucture and PartIcles in general ( this the the major use in particle physics)
  • The source of cosmic rays is of intrest to astronomers and cosmologists (see the University of Birmingham Astrophysics web page: )
  • Radiocarbon dating resulted from cosmic ray research (see )

Types of Cosmic Rays

As mentioned before we can get two forms of cosmic rays, primary and secondary. Primary cosmic rays are the rays that are formed initially after a supernova and are normally Hydrogen or Helium nuclei. However it is rare to detect a primary cosmic ray at ground level as they usually undergo collisions with atoms high up in the atmosphere, it is these collisions that create a cascade particles that are the secondary cosmic rays. These secondary cosmic rays are composed of pions, muons (the particle a spark chamber detects) and neutrinos.

Are cosmic rays harmful?

Primary cosmic rays could, potentially cause damage to cells in the body, and can cause cancer. Some scientists also have a theory, that cosmic rays may have altered Man's evolution, by altering DNA. However, it is highly unlikely that the primary rays will reach ground level (although it is not unknown), they will have already collided in the upper atmosphere. Secondary rays, which are common around ground level, are not harmful. If they were, or if the upper atmosphere didn't stop the primary rays, life on Earth may not have been possible.

Further Reading

For anyone intrested in finding out more about cosmic rays chose from one of the recommended sites below, happy reading

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