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Cosmic rays

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Cosmic rays were first considered after hot air balloon flights by the Austrian Victor Hess in 1911 and 1912 showed that air becomes more electrically conductive with altitude and reasoned that the effect is probably not from terrestrial causes. [1] The Italian, Domenico Pacini simultaneously showed that the effect decreased with depth of water with the same conclusion (that the effect is not terrestrial in origin), though only Victor Hess received the Nobel Prize in 1934[2]. Robert Millikan thought this effect came from electromagnetic waves like gamma rays and named the cause Cosmic Rays, though later, since they were deflected by magnetic fields, cosmic rays were found to be charged atomic particles rather than gamma rays or x-rays[3]. Cosmic background radiation which some think was left over from the big bang are photons (at microwave wavelengths), and thus not cosmic rays (which are high speed atomic particles).[4]

Before particle accelerators were made, cosmic rays were the main source of high energy particles for research, and figured in the discovery of muons and positrons[5]. They are now being investigated as clues to stellar nucleosynthesis, indicators for the content of interstellar gas, indicators of the size of the sun's magnetic field, possible factors for global warming and cooling, and even as a way to calculate the water content of soils.[6]

Makeup of Cosmic Rays

Most cosmic rays are protons and alpha particles (sometimes referred to as Hydrogen and Helium because they are the nuclei of these atoms). Some particles, which are called galactic cosmic rays, are believed to be released by supernovas and accelerated to high speeds while others, called solar cosmic rays, are simply released from solar flares. Particles from supernovas are thought to create other particles when they strike atoms in interstellar space, these are called secondary cosmic rays[7]. About one percent of galactic cosmic rays have nuclei of heavier atoms, and research into their proportions and energies has become a branch of astronomy.

When cosmic rays hit the earth's upper atmosphere they strike atoms in the air with enough force to create new particles which make up most of the charged particles that reach the ground. These air shower particles are also sometimes called secondary cosmic rays[8] which can cause confusion for readers because some galactic particles are called secondary cosmic rays. The number of these air shower particles is highest at about 15 kilometers above the ground, but is reduced to about one twentieth of that highest number at sea level because of the protective absorption of air molecules[9]. People who fly regularly in commercial aircraft have a slightly higher exposure to radiation because they are above the thicker part of the atmosphere and so enjoy less protection.

In 1990, an analysis of satellite data gave the following abundances of cosmic rays averaged for the energies of .6 Gev to 35 Gev. The information was normalized in terms of 100 silicon nuclei.[10] They assumed that the interstellar medium was 90% Hydrogen and 10% Helium, and that the cosmic rays of these energies all originated from other stars than our sun.

Element Nucleus Number of Nuclei per 100 Silicon nuclei Atoms in the Sun per 100 Silicon atoms[11]
Carbon 424.9 1,000
Nitrogen 25.4 310
Oxygen 526.3 2,400
Neon 58.0 300
Sodium 3.23 6.0
Magnesium 103.8 100
Aluminum 7.78 8.3
Silicon 100.0 100
Phosphorus .77 .8
Sulfur 13.1 45
Argon 2.23 10
Potassium .40 .37
Calcium 6.01 6.4
Iron 100.8 90
Cobalt 0.19 .23
Nickel 5.68 5.0

Most cosmic rays follow curved paths because of the galactic magnetic field so no one is sure where they come from. However, some very high energy cosmic rays that are thought to come from other galaxies are believed to follow paths that allow astronomers to trace them back to their source. It was assumed that almost all of the very high cosmic rays were protons (hydrogen nuclei), but a recent find shows that a surprising quantity of these rays fit the models best if they are actually iron nuclei. [12][13]

Cosmic Ray Energies

Energies of cosmic rays, like energies of nuclear particles, are measured in electronvolts (abbreviated eV).[14] Joules measure energy or work done, if you push one coulomb of electric charge into a capacitor sized so that its voltage comes up to one volt, you have a joule of energy stored in the capacitor[15]. The amount of energy that each electron of that coulomb stores is one electronvolt of energy. This measure of energy is used for atomic particles because it is easier than using minute fractions of a joule and is equal to 1.60218 x 10-19 joules. Like a joule, it can represent energy, work, or quantity of heat[16], so, electronvolts can also be a measure of the temperature of molecules. Electronvolts are even used for representing a particle's mass (but you must assume that their mass is divided by the speed of light squared which is defined as 1 for this case). It takes about 35 electron volts of energy to ionize one atom of hydrogen by removing its electron.

Particle energy source Energy by prefix Energy by powers of ten Reference
Energies of individual molecules of room temperature air .03 eV 10-2 [17]
Solar Cosmic Rays less than 1 MeV 106
The first cyclotron in 1931-2 accelerated protons 1.2 MeV 106 [18]
Anomalous Cosmic Rays (thought to originate at the edge of the solar system) about 10 MeV 107
Galactic Cosmic Rays (probably from Supernovas) 1 GeV 109
Most energetic Cosmic Ray recorded (1990) 3 x 1020 [19]

Cosmic Ray Detectors

There were originally two main types of detectors, cloud chambers and spark chambers, both relying on the tendency of a cosmic ray and its secondary particles to ionize air molecules.

See Also


  1. Penetrating Radiation at the Surface of and in Water by Domenico Pacini; translated, commented by Alessandro De Angelis
  2. Penetrating Radiation at the Surface of and in Water by Domenico Pacini; translated, commented by Alessandro De Angelis
  3. H. Svensmark and Nigel Calder, The Chilling Stars - A New Theory of Climate Change (Icon Books Ltd, 2007), 35.
  4. Cosmicopia: an Abundance of Cosmic Rays, by the Cosmic Ray Group in the Astrophysics Science Division at NASA Goddard Space Flight Center
  5. Cosmic Rays R. A. Mewaldt, California Institute of Technology
  6. Zreda, M., D. Desilets, T. P. A. Ferré, and R. L. Scott (2008), Measuring soil moisture content non-invasively at intermediate spatial scale using cosmic-ray neutrons, Geophys. Res. Lett., 35, L21402, doi:10.1029/2008GL035655
  7. Cosmic Ray Energetics And Mass (CREAM) Overview by E. S. Seo et al, 30th International Cosmic Ray Conference, 2007
  8. Cosmic Rays R. A. Mewaldt, California Institute of Technology
  9. H. Svensmark and Nigel Calder, The Chilling Stars - A New Theory of Climate Change (Icon Books Ltd, 2007), 52.
  10. Charge composition and energy spectra of cosmic-ray nuclei for elements from Be to Ni. Results from HEAO-3-C2 by Engelmann, Ferrando, Soutoul, Goret, Juliusson, Kock-Miramond, Lund, Masse, Peters, Petrou, Rasmussen Astron. Astrophys. 233, 96-111 (1990) This research has made use of NASA's Astrophysics Data System Bibliographic Services.
  11. National Physics Laboratory Kaye and Laby Tables of Physical and Chemical constants, Sun values taken from N. Grevesse and E. Anders (1988) in Cosmic Abundances of Matter (ed. J. Waddington), Amer. Inst. Phys., New York, p. 1.
  12. Iron-ic twist deepens cosmic ray puzzle Researchers present new findings about the most energetic charged particles in the universe By Ron Cowen Science News Web edition : Tuesday, June 23rd, 2009
  13. Studies of Cosmic Ray Composition And Air Shower Structure With The Pierre Auger Observatory, Presentations for the 31st International Cosmic Ray Conference, Lodz, Poland, July 2009.
  14. Einstein Online Dictionary
  15. Science Hobbyist by Bill Beatty
  16. NIST Reference on Constants, Units, and Uncertainty, of the Fundamental Constants Data Center of the NIST Physics Laboratory
  17. The Exploration of the Earth's Magnetosphere: An educational web site by David P. Stern and Mauricio Peredo
  18. Cern Teachers Resources - Accelerators, Accessed June 15, 2009
  19. Cosmicopia: an Abundance of Cosmic Rays, by the Cosmic Ray Group in the Astrophysics Science Division at NASA Goddard Space Flight Center

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