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Cosmic raysÂ are energetic particles originating fromÂ outer spaceÂ that impinge onÂ Earth's atmosphere. About 89% of all the incoming cosmic ray particles are simpleÂ protons, with nearly 10% beingÂ heliumÂ nuclei (alpha particles), and slightly under 1% are heavier elements;Â electronsÂ (beta particles) constitute about 1% of galactic cosmic rays.Â The termÂ rayÂ is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles. However, when they were first discovered, cosmic rays were thought to be rays. When theirÂ particleÂ nature needs to be emphasized, "cosmic ray particle" is written.
The variety of particle energies reflects the wide variety of sources. The origins of these particles range from energetic processes on theÂ SunÂ all the way to as yet unknown events in the farthest reaches of the visibleÂ universe. Cosmic rays can have energies of over 1020Â eV, far higher than the 1012Â to 1013Â eV that man-made particle accelerators can produce. (SeeÂ Ultra-high-energy cosmic raysÂ for a description of the detection of a single particle with an energy of about 50Â J, the same as a well-hit tennis ball at 42Â m/s [about 150Â km/h].) There has been interest in investigating cosmic rays of even greater energies.
Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic rays that arise in extrasolar astrophysical sources are primary cosmic rays; these primary cosmic rays can interact withÂ interstellar matterÂ to create secondary cosmic rays. The Sun also emits low energy cosmic rays associated withÂ solar flares. The exact composition of primary cosmic rays, outside theÂ Earth's atmosphere, is dependent on which part of theÂ energy spectrumÂ is observed. However, in general, almost 90% of all the incoming cosmic rays areÂ protons, about 9% areÂ heliumÂ nuclei (alpha particles) and nearly 1% areÂ electrons. The ratio of hydrogen to helium nuclei (28% helium by mass) is about the same as the primordialelemental abundanceÂ ratio of these elements (24% by mass He) in the universe.
The remaining fraction is made up of the other heavier nuclei which are abundant end products of stars' nuclear synthesis. Secondary cosmic rays consist of the other nuclei which are not abundant nuclear synthesis end products, or products of theÂ Big Bang, primarilyÂ lithium,Â beryllium, andÂ boron. These light nuclei appear in cosmic rays in much greater abundance (about 1:100 particles) than in solar atmospheres, where their abundance is about 10âˆ’7Â that ofÂ helium.
This abundance difference is a result of the way secondary cosmic rays are formed. When the heavy nuclei components of cosmic rays, namely the carbon and oxygen nuclei, collide with interstellar matter, they break up into lighter nuclei (in a process termedÂ cosmic ray spallation) - lithium, beryllium and boron. It is found that the energy spectra of Li, Be and B fall off somewhat more steeply than those of carbon or oxygen, indicating that lessÂ cosmic ray spallationÂ occurs for the higher energy nuclei presumably due to their escape from theÂ galacticmagnetic field. Spallation is also responsible for the abundances ofÂ scandium,Â titanium,Â vanadium, andÂ manganeseÂ ionsÂ in cosmic rays, which are produced by collisions of iron and nickel nuclei withÂ interstellar matter.Â
REVIEW OF LITERATURE
After the discovery ofÂ radioactivityÂ byÂ Henri BecquerelÂ in 1896, it was generally believed that atmospheric electricity (ionizationÂ of theÂ air) was caused only byÂ radiationÂ from radioactive elements in the ground or the radioactive gases (isotopes ofÂ 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.
In 1909Â Theodor WulfÂ developed anÂ electrometerÂ (a device to measure the rate of ion production inside a hermetically sealed container) and used it to show higher levels of radiation at the top of theÂ Eiffel TowerÂ than at its base, but his paper published inÂ Physikalische ZeitschriftÂ was not widely accepted. In 1911Â Domenico PaciniÂ observed simultaneous variations of the rate of ionization over a lake, over the sea, and at a depth of 3 meters from the surface. Pacini concluded from the decrease of radioactivity underwater that a certain part of the ionization must be due to sources other than the radioactivity of the Earth.Â Then, in 1912,Â Victor HessÂ carried three enhanced-accuracy Wulf electrometersÂ built in 1911 to an altitude of 5300 meters in aÂ free balloonÂ flight. He found the ionization rate increased approximately fourfold over the rate at ground level.Â Hess also ruled out the Sun as the radiation's source by making a balloon ascent during a near-total eclipse. With the moon blocking much of the Sun's visible radiation, Hess still measured rising radiation at rising altitudes.Â 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." In 1913-1914,Â Werner Kolhörsterconfirmed Victor Hess' earlier results by measuring the increased ionization rate at an altitude of 9Â km. Hess received theÂ Nobel Prize in PhysicsÂ in 1936 for his discovery.
The term "cosmic rays" was coined byÂ Robert MillikanÂ who proved they were extraterrestrial in origin, and not produced by atmospheric electricity. Millikan believed that cosmic rays were high-energyÂ photonsÂ with some secondaryÂ electronsÂ produced byÂ Compton scatteringÂ of gamma rays. Compton himself held the (correct) belief that cosmic rays were primarily charged particles. During the decade from 1927 to 1937, a wide variety of experimental investigations demonstrated that the primary cosmic rays are mostly positively 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,Â muons. The muon was initially believed to be the unstable particle predicted byÂ Hideki YukawaÂ in 1935 in his theory of theÂ nuclear force. Experiments proved that the muon decays with aÂ mean lifeÂ of 2.2 microseconds into an electron and twoÂ neutrinos, but that it does notÂ interact stronglyÂ withÂ nuclei, so it could not be the Yukawa particle. The mystery was solved by the discovery in 1947 of theÂ 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 pionâ†’muonâ†’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 byÂ GottliebÂ andÂ Van AllenÂ showed that the primary cosmic particles are mostlyÂ protonsÂ with some helium nuclei (alpha particles) and a small fraction heavier nuclei.
In 1934,Â Bruno RossiÂ reported an observation of near-simultaneous discharges of twoÂ Geiger countersÂ widely separated in a horizontal plane during a test of equipment he was using in a measurement of the so-calledÂ 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, he did not have the time to study this phenomenon more closely." In 1937Â Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that extensiveÂ particle showersÂ 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.
Attempts were made to measure the primary cosmic ray component at very high altitude. Soviet physicistÂ Sergey VernovÂ was the first to useÂ radiosondesÂ to perform cosmic ray readings at high altitude. On April 1, 1935, he took measurements up to 13.6 kilometers using a pair of geiger counters in an anti-coincidence circuit to avoid counting secondary ray showers.
Homi J. BhabhaÂ derived an expression for the probability of scattering positrons by electrons, a process now known as Bhabha scattering. His classic paper, jointly withÂ Walter Heitler, published in 1937 described how primary cosmic rays from space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler explained the cosmic ray shower formation by the cascade production of gamma rays and positive and negative electron pairs. In 1938 Bhabha concluded that observations of the properties of such particles would lead to the straightforward experimental verification of Albert Einstein's theory of relativity.
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 theÂ Massachusetts Institute of Technology. The experiment employed elevenÂ scintillation detectorsÂ arranged within a circle 460 meters in diameter on the grounds of the Agassiz Station of theÂ Harvard College Observatory. From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 1020Â eV (past theÂ GZK cutoff, beyond which very few cosmic rays should be observed). A huge air shower experiment called theÂ Auger ProjectÂ is currently operated at a site on theÂ pampasÂ 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. In November, 2007 preliminary results were announced showing direction of origination of the 27 highest energy events were strongly correlated with the locations ofÂ active galactic nucleiÂ [AGN], where bare protons are believed accelerated by strong magnetic fields associated with the largeÂ black holesÂ at the AGN centers to energies of 1020Â eV and higher.
Three varieties ofÂ neutrinoÂ are produced when the unstable particles produced in cosmic ray showers decay. Since neutrinos interact onlyÂ weaklyÂ 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. TheSuper-KamiokandeÂ in Japan provided the first convincing evidence forÂ neutrino oscillationÂ in which oneÂ flavourÂ 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.
TheÂ Moon's cosmic ray shadow, as seen in secondary muons detected 700 m below ground, at theÂ Soudan 2Â detector.
The Moon as seen by theÂ Compton Gamma Ray Observatory, in gamma rays of greater than 20 MeV. These are produced by cosmic ray bombardment of its surface. The Sun, which has no similar surface of highÂ atomic numberÂ to act as target for cosmic rays, cannot be seen at all at these energies, which are too high to emerge from primary nuclear reactions, such as solar nuclear fusion.
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 aÂ shower, result in the production of manyÂ pionsÂ andÂ kaons, unstableÂ mesonsÂ which quickly decay intoÂ muons.
Because muons do not interact strongly with the atmosphere, and because of the relativistic effect ofÂ time dilationÂ in the Earth's reference frame (alternately, length contraction in the muon's reference frame) many of these muons are able to reach the surface of the Earth and even penetrate for some distance into shallow mines. Muons areÂ ionizing radiation, and may easily be detected by many types of particle detectors such asÂ cloud chambersÂ orÂ bubble chambersÂ orÂ scintillationÂ 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.
Cosmic rays impacting other (non-Earth) bodies in the Solar System which are made of elements heavier than hydrogen and helium, can be detected indirectly by observing high energy gamma ray emissions from these bodies using a gamma-ray telescope (see image at right). When such gammas are of energy too high to result from radioactive decay processes (> about 10 MeV) they must be secondary to cosmic ray bombardment.
Detection by particle track-etch technique
Cosmic rays can also be detected directly when they pass through particle detectors flown aboard satellites or in high altitude balloons. In a pioneering technique developed byÂ Robert Fleischer,Â P. Buford Price, andÂ Robert M. Walker,Â sheets of clear plastic such as 1/4 milÂ Lexanpolycarbonate can be stacked together and exposed directly to cosmic rays in space or high altitude. When returned to the laboratory, the plastic sheets are "etched" [literally, slowly dissolved] in warm causticÂ sodium hydroxideÂ solution, which removes the surface material at a slow, known rate. Wherever a bare cosmic ray nucleus passes through the detector, the nuclear charge causes chemical bond breaking in the plastic. The slower the particle, the more extensive is the bond-breaking along the path; and the higher the charge (the higher the Z), the more extensive is the bond-breaking along the path. The caustic sodium hydroxide dissolves at a faster rate along the path of the damage, but thereafter dissolves at the slower base-rate along the surface of the minute hole that was drilled. The net result is a conical shaped pit in the plastic; typically with two pits per sheet (one originating from each side of the plastic). The etch pits can be measured under a high power microscope (typically 1600X oil-immersion), and the etch rate plotted as a function of the depth in the stack of plastic. At the top of the stack, the ionization damage is less due to the higher speed. As the speed decreases due to deceleration in the stack, the ionization damage increases along the path. This generates a unique curve for each atomic nucleus of Z from 1 to 92, allowing identification of both the charge and energy (speed) of the particle that traverses the stack. This technique has been used with great success for detecting not only cosmic rays, but fission product nuclei for neutron detectors.
Â Interaction with the Earth's atmosphere
When cosmic ray particles enter theÂ Earth's atmosphereÂ they collide withÂ molecules, mainly oxygen and nitrogen, to produce a cascade of lighter particles, a so-calledÂ air shower. The general idea is shown in the figure which shows a cosmic ray shower produced by a high energy proton of cosmic ray origin striking an atmospheric molecule.
All of the produced particles stay within about one degree of the primary particle's path. Typical particles produced in such collisions are chargedÂ mesonsÂ (e.g. positive and negativeÂ pionsÂ andÂ kaons). Cosmic rays are also responsible for the continuous production of a number ofÂ unstable isotopesÂ in the Earth's atmosphere, such asÂ carbon-14, via the reaction:
n + N14 P + C14
Cosmic rays kept the level ofÂ carbon-14Â in the atmosphere roughly constant (70 tons) for at least the past 100,000 years, until the beginning of above-ground nuclear weapons testing in the early 1950s. This is an important fact used inÂ radiocarbon datingÂ which is used inÂ archaeology.
Reaction products of secondary cosmic ray, lifetime and reaction
EFFECTS OF COSMIC RAYS
Changes in Atmospheric Chemistry
Cosmic rays ionize the nitrogen and oxygen molecules in the atmosphere, which leads to a number of chemical reactions. One of the reactions result in ozone depletion. The magnitude of damage, however, is very small compared to the depletion caused by CFCs.
Role in ambient radiation
Cosmic rays constitute a fraction of the annual radiation exposure of human beings on the Earth. For example, the average radiation exposure in Australia is 0.3Â mSvÂ due to cosmic rays, out of a total of 2.3 mSv.
Effect on electronics
Cosmic rays have sufficient energy to alter the states of elements inÂ electronicÂ integrated circuits, causing transient errors to occur, such as corrupted data inÂ electronic memory devices, or incorrect performance ofÂ CPUs, often referred to as "soft errors" (not to be confused with software errors caused by programming mistakes/bugs). This has been a problem in extremely high-altitudeÂ electronics, such as inÂ satellites, but withÂ transistorsÂ becoming smaller and smaller, this is becoming an increasing concern in ground-level electronics as well. Studies byÂ IBMÂ in the 1990s suggest that computers typically experience about one cosmic-ray-induced error per 256 megabytes ofÂ RAMÂ per month.
To alleviate this problem, theÂ Intel CorporationÂ has proposed a cosmic ray detector that could be integrated into future high-densityÂ microprocessors, allowing the processor to repeat the last command following a cosmic-ray event.
Cosmic rays were recently suspected as a possible cause of aÂ Qantas AirlinesÂ in-flight incident where an Airbus A330Â airlinerÂ twice plunged hundreds of feet after an unexplained malfunction in its flight control system. Many passengers and crew members were injured, some seriously. After this incident, the accident investigators determined that the airliner's flight control system had received a data spike that could not be explained, and that all systems were in perfect working order. This has prompted a software upgrade to all A330 & A340 airliners, worldwide, so that any data spikes in this system are filtered out electronically.Â
Significance to space travel
Galactic cosmic raysÂ are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft. Cosmic Rays also place a threat to electronics placed aboard outgoing probes. A recent malfunction aboard theÂ Voyager 2Â space probe was credited to a single flipped bit, probably caused by a cosmic ray.
Role in lightning
Cosmic rays have been implicated in the triggering of electrical breakdown inÂ lightning. It has been proposed 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.
Role in climate change
A role of cosmic rays directly or via solar-induced modulations in climate change was suggested byÂ E.P.Ney in 1959Â and byÂ Robert Dickinson in 1975. In recent years, the idea has been revived most notably byÂ Henrik Svensmark; the most recent IPCC studyÂ disputedÂ the mechanism,Â while the most comprehensive review of the topic to date states: "evidence for the cosmic ray forcing is increasing as is the understanding of its physical principles".