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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Cosmos

Cosmic Rays

The so-called cosmic-ray radiation is high-energy particle radiation from the cosmos which was discovered by Viktor Hess in 1912 with balloon experiments (Nobel prize in 1936).  This radiation contains mainly protons and completely ionized atoms and electrons.  The energy spectrum of cosmic radiation can be described by a power law what is evidence for its origin as non-thermal radiation.  The energy of some particles from the cosmic-ray radiation is about 100 million times higher than the radiation produced by modern particle colliders on Earth.  The energy of the so called “Oh My God” particle, a proton detected by the Fly's Eye experiment, was 51 Joule, equivalent, in the words of Utah physicist Pierre Sokolsky, to “a brick falling on your toe”.

The detection of cosmic rays is made by spectrometers on satellites or in balloons above the Earth's atmosphere.  By interactions of primary particles of the cosmic radiation with gas molecules in the atmosphere, particle showers are produced which contain a high number of secondary particles; protons, neutrons, photons, electrons and muons of which only a fraction reaches the Earth's surface and can be used for ground-based indirect measurement of the flux of cosmic radiation.  The radioactive contamination by cosmic radiation is about 10 % of the annual natural radiation level in Germany (2 mSv).

Even 100 years after the discovery of cosmic rays the search for its sources still continues.  So far we know that cosmic rays of low energy (<1 GeV) come from the Sun.  By using satellite experiments, Imaging Cherenkov telescopes (like H.E.S.S.) and neutrino telescopes scientists try to find out if supernova remnants could be sources of the galactic cosmic rays.  Cosmic rays of the highest energies are considered to be of extragalactic origin: the size of the Milky Way and its magnetic field do not allow particles to be accelerated to such high energies.  It is believed that the origin of the highest energy cosmic rays are nearby active galaxies and/or mysterious gamma-ray bursts.


Cosmos | Supernovae | Pulsars | Galaxies | Cosmic Rays | γ-Astronomy