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. The illustration at right shows that the detected cosmic ray flux peaks at about 15 km in altitude and then drops sharply (note the logarithmic scale on the altitude). This kind of variation was discovered by Pfotzer in 1936. It suggests that the detection method used was mainly detecting secondary particles rather than the primary particles reaching the Earth from space.

Analysis of the particle populations in cosmic rays yields hints about their origin.

Composition of cosmic raysThe solar wind
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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 abundance of these elements in the universe which is only about one billionth! From this evidence it is implied that these light elements have been produced as fragments in high-speed collisions when primary cosmic ray particles like protons strike elements like carbon and oxygen in the very tenuous matter in interstellar space. Attempts have been made to model how much ordinary matter would be required along their pathway for collisions to produce the observed population of these light elements. One study suggested that it is about equivalent to passing through 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.

One interesting aspect of cosmic rays is that they are almost totally matter rather than antimatter. According to Carroll & Ostlie, only about 0.01% of cosmic rays are antimatter, so this sample of the particles of our galaxy provides evidence of the matter-antimatter asymmetry in our galaxy and presumably in the universe as a whole. The few antiparticles that are observed can be accounted for as the results of high energy particle collisions that produce particle-antiparticle pairs.

High energy collisions in the upper atmosphere produce cascades of lighter particles. Pions and kaons 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)


Chaisson & McMillan
Sec 23.7

Carroll & Ostlie
Sec 30.1

NASA, Cosmic Ray Composition
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