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Extremely energetic atomic nuclei which travel through the Universe at practically
the speed of light and strike the Earth from all direction. Almost 90% of all the
incoming primary cosmic rays are protons, about 9% are helium nuclei (alpha particles)
and about 1% are electrons (beta minus particles). Some cosmic rays come from the
Sun (mainly due to solar flares), most come from galactic supernovae, and a few with
the highest energy are suspected to originate from outside the Milky Way. As for their
flux, about 1 charged particle per second per cm^2 impacts the Earth. The typical
kinetic energy of these particles is about 10 MeV/nucleon to several GeV/nucleon,
although there are some at higher energies. In fact, the cosmic ray with the highest
energy has been measured above x 10^20 eV. These ultra-high energy cosmic rays are
suspected to be extragalactic, as there is no plausible mechanism of acceleration
to these energies by a supernova, for example. Again, compare these energies to those
of solar neutrinos that have only 0.26 MeV. Cosmic rays may be divided into primary
cosmic rays and secondary cosmic rays. Their energy ranges from 10^9 to 10^20 electron-volts.
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