The Online Encyclopedia and Dictionary






Cosmic ray

Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. The composition includes electrons, protons, gamma rays, and atomic nuclei from a large region of the periodic table. The kinetic energies of these particles span over fourteen orders of magnitude, with the flux of cosmic rays on the Earth's surface falling approximately as the inverse cube of the energy. The wide variety of particle energies is reflected in the wide variety of sources. Cosmic rays originate from energetic processes on the Sun all the way to the farthest reaches of the visible universe.

Cosmic rays can be conceptually broken into different kinds:

Cosmic rays can have energies up to 1020 eV 1

History of cosmic rays

Cosmic rays, also known as cosmic particles, were initially believed to originate in radioactive isotopes in the ground. This theory was disproven in 1911 by Victor Hess, who in 1936 received the Nobel prize in physics for his work. Hess used electroscope measurements taken at different altitudes from a hot air balloon to conclude that the radiation was cosmic in origin. Hess further showed that the sun could not be the primary source of cosmic rays by taking balloon measurements during a 1912 solar eclipse.

In 1938, Pierre Auger observed near-simultaneous cosmic ray events at widely separated locations. He concluded that they were due to incident particles whose energy was too high to penetrate the atmosphere. Such particles instead collide with nuclei in the atmosphere, initiating a particle cascade known as a cosmic ray air shower. The events Auger had observed were found to have energies of 1015 eV, 10 million times higher than had previously been known.

The measurement of high-energy cosmic rays via sampling of extended air showers was first implemented in 1954 at the Harvard College Observatory. From their work, and from the many ground-array experiments that followed it, the cosmic ray spectrum is now known to extend up to at least 1020 eV.


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