Cosmic Rays

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What is a Cosmic Ray?

Cosmic Rays are actually individual energized particles that originate from space, not rays as the name would suggest. These particles essentially include all of the elements in the periodic table.

Almost 90% of the particles are Protons (hydrogen), about 9% are Helium Nuclei and 1% are Electrons(other heavier elements). They were discovered in 1912 by Victor Hess as he ascended in a balloon with an electroscope. He found that it discharged more rapidly as he ascended. He attributed this to a source of radiation entering the atmosphere from above. In 1936 he was awarded the Nobel prize for his discovery. It was believed for a while that the radiation was electromagnetic in nature hence the name cosmic “rays”. cosmic rays are still incorrectly included as part of the electromagnetic spectrum in some textbooks. In the 1930’s it was found that because cosmic rays are affected by the Earth’s magnetic field they must be electrically charged.

During the period from the 1930s to the 1950s (before man-made particle accelerators reached very high energies) cosmic rays served as a source of particles for high energy physics investigations. This led to the discovery of subatomic particles that included the positron and muon. Although these applications continue the main focus of cosmic ray research is now directed towards astrophysical investigations of where cosmic rays originate, how they get accelerated to such high velocities, what role they play in the dynamics of the Galaxy, and what their composition tells us about matter from outside the solar system.

Research into cosmic rays is now carried out by instruments carried on spacecraft and high altitude balloons, using particle detectors similar to those used in nuclear and high energy physics experiments.

It is believed that most galactic cosmic rays get their energy from supernova explosions. There is considerable evidence that cosmic rays are accelerated as the shock waves from these explosions travel through the surrounding interstellar gas. But because cosmic rays are electrically charged they are deflected by magnetic fields, and their directions have been randomized, making it impossible to tell where they originated. But some sources can be traced. The Crab Nebula Supernova remnant is known to be a source of cosmic rays from the radio synchrotron radiation emitted by cosmic ray electrons spiraling in the magnetic fields of the remnant.

The energy that cosmic rays contribute to the Galaxy is about equal to that contained in galactic magnetic fields, and in the thermal energy of the gas that is present in the space between the stars.


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