Cosmic ray
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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 bare 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.
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.Template:Ref
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Detection
The 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. 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
After the discovery of radioactivity by Henri Becquerel in 1896, it was generally believed that atmospheric electricity (ionization of the air) was caused only by radiation from radioactive elements in the ground or the radioactive gases (isotopes of radon) they produce. Measurements of ionization rates at increasing heights above the ground during the decade from 1900 to 1910 showed a decrease that could be explained as due to absorption of the ionizing radiation by the intervening air. Then, in 1912, Victor Hess carried three Wulf electrometers (a device to measure the rate of ion production inside a hermetically sealed container) to an altitude of 5300 meters in a free balloon flight. He found the ionization rate increased approximately four-fold over the rate at ground level. He concluded "The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above." Hess received the Nobel Prize in Physics in 1936 for his discovery of what came to be called "cosmic rays".
For many years it was generally believed that cosmic rays were high-energy photons (gamma rays) with some secondary electrons produced by Compton scattering of the gamma rays. Then, during the decade from 1927 to 1937 a wide variety of experimental investigations demonstrated that the primary cosmic rays are mostly positive charged particles, and the secondary radiation observed at ground level is composed primarily of a "soft component" of electrons and photons and a "hard component" of penetrating particles, muons. The muon was initially believed to be the unstable particle predicted by Hideki Yukawa in 1935 in his theory of the nuclear force. Experiments proved that the muon decays with a mean life of 2.2 microseconds into an electron and two neutrinos, but that it does not interact strongly with nuclei, so it could not be the Yukawa particle. The mystery was solved by the discovery in 1947 of the pion, which is produced directly in high-energy nuclear interactions. It decays into a muon and one neutrino with a mean life of 0.0026 microseconds. The pion→muon→electron decay sequence was observed directly in a microscopic examination of particle tracks in a special kind of photographic plate called a nuclear emulsion that had been exposed to cosmic rays at a high-altitude mountain station. In 1948 observations with nuclear emulsions carried by balloons to near the top of the atmosphere Gottlieb and Van Allen showed that the primary cosmic particles are mostly protons with some helium nuclei (alpha particles) and a small fraction heavier nuclei.
In 1934 Bruno Rossi reported an observation of near-simultaneous discharges of two Geiger counters widely separated in a horizontal plane during a test of equipment he was using in a measurement of the so-called east-west effect. In his report on the experiment, Rossi wrote "...it seems that once in a while the recording equipment is struck by very extensive showers of particles, which causes coincidences between the counters, even placed at large distances from one another. Unfortunately, I did not have the time to study this phenomenon more closely." In 1937 Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that extensive particle showers are generated by high-energy primary cosmic-ray particles that interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, photons, and muons that reach ground level.
Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of "density sampling" and "fast timing" of extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the Massachusetts Institute of Technology. The experiment employed eleven scintillation detectors arranged within a circle 460 meters in diameter on the grounds of the Agassiz Station of the Harvard College Observatory. From that work, and from many other experiments carried out by MIT and by other groups in the US, England, Japan, and Russia, and Bolivia, the energy spectrum of the primary cosmic rays is now known to extend beyond 1020 eV (past the GZK cutoff, beyond which very few cosmic rays should be observed). A huge air shower experiment called the Auger Project is currently operated at a site on the pampas of Argentina by an international consortium of physicists. Their aim is to explore the properties and arrival directions of the very highest energy primary cosmic rays. The results are expected to have important implications for particle physics and cosmology.
Three varieties of neutrino are produced when the unstable particles produced in cosmic ray showers 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 in Japan provided the first convincing evidence for neutrino oscillation in which one flavour of neutrino changes into another. The evidence was found in a difference in the ratio of electron neutrinos to muon neutrinos depending on the distance they have traveled through the air and earth.
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[1].
Due to the potential negative effects of astronaut exposure to cosmic rays, solar activity may play a role in future space travel via the Forbush decrease effect. Coronal mass ejections (CMEs) can temporarily lower the local cosmic ray levels, and radiation from CMEs is easier to shield against than cosmic rays.
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.
Research
There are a number of cosmic ray research initiatives. These include, but are not limited to:
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
References
- Template:Note 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.
See also
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.ca:Raigs còsmics
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