Cosmic Rays

Cosmic ray is the term given to high energy radiation which strikes the Earth from space. Some of them have ultrahigh energies in the range 100 - 1000 TeV. Such extreme energies come from only a few sources like Cygnus X-3. The peak of the energy distribution is at about 0.3 GeV.

The intensity of cosmic radiation increases with altitude, indicating that it comes from outer space. It changes with latitude, indicating that it consists at least partly of charged particles which are affected by the earth's magnetic field. Analysis of the particle populations in cosmic rays yields hints about their origin.

Composition of cosmic rays
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Particles in Cosmic Rays

Almost 90% of the cosmic rays which strike the Earth's atmosphere are protons (hydrogen nuclei) and about 9% are alpha particles. Electrons amount to about 1% according to Chaisson & McMillan. There is a small fraction of heavier particles which yield some interesting information. About 0.25% are light elements (lithium, beryllium and boron), but this is greatly enriched over the abundunce of these elements in the universe which is only about one billionth! From this evidence it is implied that these light elements come from the bombardment of heavier particles. Statistical analyses of the light particles point to passage of primary cosmic rays through an amount of matter equivalent to about 4 cm of water.

Medium elements (carbon, nitrogen, oxygen and flourine) are about 10 times their abundance in normal matter and the heavier elements are increased about a hundredfold over normal matter. This suggests an origin of cosmic rays in areas of space with greatly enriched amounts of heavy elements. The density of cosmic rays in interstellar space is estimated to be about 10-3/m3.

High energy collisions in the upper atmosphere produce cascades of lighter particles. Pions are produced, which decay to produce muons. Muons make up more than half of the cosmic radiation at sea level, the remainder being mostly electrons, positrons and photons from cascade events.(Richtmyer)

Index

Reference
Chaisson & McMillan
Sec 23.7
 
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