A cosmic ray is a high-speed particle - either an atomic nucleus or an electron - that travels throughout the Milky Way Galaxy, including the solar system. Some of these particles originate from the Sun, but most come from sources outside the solar system and are known as galactic cosmic rays (GCRs).

- britannica.com

• The Berkeley Lab Cosmic Ray Telescope Project
  

In astrophysics, cosmic rays are radiation consisting of energetic particles originating beyond the Earth that impinge on the Earth's atmosphere. Cosmic rays are composed mainly of ionized nuclei, roughly 87% protons, 12% alpha particles (helium nuclei) and most of the rest being made up of heavier atomic nuclei. Electrons, gamma rays, and very high-energy neutrinos also make up a much smaller fraction of the cosmic radiation.

energy spectrum for cosmic ray nuclei

The kinetic energies of cosmic ray 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 reflects 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 have energies up to 10²⁰ eV (see Oh-My-God particle for the first recorded event of a particle of such high energy). There has been interest in investigating cosmic rays of even greater energies.^[1]

Detection

The ionized nuclei that make up cosmic rays are able to travel from their distant sources to the Earth because of the low density of matter in space. Charged nuclei interact strongly with other matter, so when the cosmic rays approach Earth they begin to collide with the nuclei of atmospheric gases. These collisions, in a process known as a shower, result in the production of many pions and kaons, unstable mesons which quickly decay into muons. Because muons do not interact strongly with the atmosphere and because of the relativistic effect of time dilation many of these muons are able to reach the surface of the Earth. Muons are ionizing radiation, and may easily be detected by many types of particle detectors such as bubble chambers or scintillation detectors. If several muons are observed by separated detectors at the same instant it is clear that they must have been produced in the same shower event.

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 1912 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.

Particle physicists thought that they had discovered Yukawa's theoretical pion in cosmic rays in the late 1930s, but quickly learned that the particle they had found had the right mass but very wrong characteristics. They had actually discovered the muon, the cosmic ray secondary particle that is most copious at the surface of the Earth. Pions interact strongly with nuclei, and because of this, they very rarely make it to the surface of the Earth. Pions were eventually discovered in mountaintop cosmic ray experiments in 1947.

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.

Neutrinos are produced when kaons and muons which were produced in cosmic ray interactions decay. Since neutrinos interact only weakly with matter most of them simply pass through the Earth and exit the other side. They very occasionally interact, however, and these atmospheric neutrinos have been detected by several deep underground experiments. The Super-Kamiokande experiment provided the first convincing evidence of neutrino oscillation by observing a direction-dependent deficit of muon neutrinos as compared to electron neutrinos.

Significance to Space Travel

Understanding the effects of cosmic rays on the body will be vital for assessing the risks of space travel. High speed cosmic rays can damage DNA, increasing the risk of cancer, cataracts, neurological disorders, and non-cancer mortality risks[2].

Time Dependence

In the past, it was believed that the cosmic ray flux has remained fairly constant over time. In fact, this is one of the fundamental assumptions behind radiocarbon dating. Recent research has, however, produced evidence for large century-timescale changes in the cosmic ray flux in the past ten thousand years (see Earth and Planetary Science Letters 234 (2005) 335-349, in particular Table 1).

Lightning

Cosmic rays have been implicated in the triggering of electrical breakdown in lightning. It has been proposed (see Gurevich and Zybin, Physics Today, May 2005, "Runaway Breakdown and the Mysteries of Lightning") that essentially all lightning is triggered through a relativistic process, "runaway breakdown", seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through "conventional breakdown" mechanisms.

Cosmic rays and fiction

Because of the metaphysical connotations of the word "cosmic", the very name of these particles enables their misinterpretation by the public, giving them an aura of mysterious powers. Were they merely referred to as "high-speed protons and atomic nuclei" this might not be so.

In fiction, cosmic rays have been used as a catchall, mostly in comics (notably the Marvel Comics group the Fantastic Four), as a source for mutation and therefore the powers gained by being bombarded with them.

Types of cosmic radiation

• Anomalous cosmic ray
• Galactic cosmic ray
• Solar cosmic ray
• Ultra-high energy cosmic ray

References

• ^  Luis Anchordoqui, Thomas Paul, Stephen Reucroft, John Swain. Ultrahigh Energy Cosmic Rays: The state of the art before the Auger Observatory. (2002) arxiv:hep-ph/0206072
• Pierre Auger Observatory: the largest cosmic ray observatory in the world, in Argentina, with a twin coming in Colorado
• Introduction to Geomagnetically Trapped Radiation by Martin Walt 1994
• A. M. Hillas, Cosmic Rays, Pergamon Press, Oxford, 1972. A good overview of the history and science of cosmic ray research including reprints of seminal papers by Hess, Anderson, Auger and others.
• B. Rossi, Cosmic Rays, McGraw-Hill, New York, 1964.
• Thomas Gaisser, Cosmic Rays and Particle Physics, Cambridge University Press, 1990.

External links

• NOAA FTP: Lal, D., et al., 2005. Data on cosmic ray flux derived from C14 concentrations in the GISP2 Greenland ice core.
• Introduction to Cosmic Ray Showers by Konrad Bernlöhr.