COSMIC RAYS

                                                                COSMIC RAYS\














Cosmic rays are very high-energy particles, mainly originating outside the Solar System. They may produceshowers of secondary particles that penetrate and impact Earth's atmosphere and sometimes even Earth's surface. Comprised primarily of high-energy protons and atomic nuclei, their origin has, until recently, been a mystery. With data from the Fermi space telescope published in February 2013,it is now known that cosmic rays primarily originate from the supernovae of massive stars, with each explosion producing roughly 3 × 10⁴² - 3 × 10⁴³ J of cosmic rays.

The term ray is an historical accident as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In modern common usage  high energy particles with intrinsic mass are known as "cosmic" rays, and photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names of gamma rays or x-rays depending on their frequencies.

Cosmic rays attract great interest practically, due to the damage they inflict on microelectronics and life outside the protection of an atmosphere and magnetic field, and scientifically, because the energies of the most energetic ultra-high-energy cosmic rays (UHECRs) have been observed to approach 3 × 10²⁰ eV, about 40 million times the energy of particles accelerated by the Large Hadron Collider. At 50 J, the highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic energy of a 90-kilometre-per-hour (56 mph) baseball.

As a result of these discoveries, there has been interest in investigating cosmic rays of even greater energies.Most cosmic rays, however, do not have such extreme energies; the energy distribution of cosmic rays peaks at 0.3 gigaelectronvolts (4.8×10⁻¹¹ J).

Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the nuclei (stripped of their electron shells) of well-known atoms, and about 1% are solitary electrons (similar to beta particles). Of the nuclei, about 90% are simple protons, i. e. hydrogen nuclei; 9% are alpha particles, and 1% are the nuclei of heavier elements. A very small fraction are stable particles of antimatter, such as positrons or antiprotons, and the precise nature of this remaining fraction is an area of active research.

History

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 or 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.

[edit]Discovery of cosmic ray

In 1909 Theodor Wulf developed an electrometer, 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 the Eiffel Tower than at its base. However, his paper published in Physikalische Zeitschrift was not widely accepted. In 1911 Domenico Pacini 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.

Then, in 1912, Victor Hess carried three enhanced-accuracy Wulf electrometers to an altitude of 5300 meters in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level. 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. 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, Werner Kolhörsterconfirmed Victor Hess' earlier results by measuring the increased ionization rate at an altitude of 9 km.

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The Hess balloon flight took place on 7 August 1912, providing the first direct evidence of cosmic radiation. By sheer coincidence, exactly 100 years later on 7 August 2012, the Mars Science Laboratory rover used its Radiation Assessment Detector (RAD) instrument to begin measuring the radiation levels on another planet for the first time.

Hess received the Nobel Prize in Physics in 1936 for his discovery.

In the 1920s the term "cosmic rays" was coined by Robert Millikan who made measurements of ionization due to cosmic rays from deep under water to high altitudes and around the globe. Millikan believed that his measurements proved that the primary cosmic rays were gamma rays, i.e., energetic photons. And he proposed a theory that they were produced in interstellar space as by-products of the fusion of hydrogen atoms into the heavier elements, and that secondary electrons were produced in the atmosphere by Compton scattering of gamma rays.

 But then, in 1927, J. Clay found evidence [J. Clay, Proc. Acad. Wetesch., Amsterdam, 30:633 (1927)], later confirmed in many experiments, of a variation of cosmic ray intensity with latitude, which indicated that the primary cosmic rays are deflected by the geomagnetic field and must therefore be charged particles, not photons. In 1929, Bothe and Kolhörster [W. Bothe & W. Kolhörster, Zeitschrift für Physik, 36:751 (1929)] discovered charged cosmic-ray particles that could penetrate 4.1 cm of gold. Charged particles of such high energy could not possibly be produced by photons from Millikan's interstellar fusion process.

[edit]Identifying the cosmic particles

In 1930, Bruno Rossi predicted a difference between the intensities of cosmic rays arriving from the east and the west that depends upon the charge of the primary particles - the so-called "east-west effect." Three independent experiments  found that the intensity is, in fact, greater from the west, proving that most primaries are positive. 

During the years from 1930 to 1945, a wide variety of investigations confirmed that the primary cosmic rays are mostly protons, and the secondary radiation produced in the atmosphere is primarily electrons, photons and muons. In 1948, observations with nuclear emulsions carried by balloons to near the top of the atmosphere [P. Freier et al., Phys. Rev. 74:213 (1948)] [H. L. Bradt & B. Peters, Phys. Rev., 74:1828 (1948)] showed that approximately 10% of the primaries are helium nuclei and 1% are heavier nuclei of the elements such as carbon, iron, and lead.

During a test of his equipment for measuring the east-west effect, Rossi observed that the rate of near-simultaneous discharges of two widely separated Geiger counters was larger than the expected accidental rate. 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."

 In 1937 Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that high-energy primary cosmic-ray particles interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, and photons that reach ground level.

Soviet physicist Sergey Vernov was the first to use radiosondes to perform cosmic ray readings with an instrument carried to high altitude by a balloon. On 1 April 1935, he took measurements at heights up to 13.6 kilometers using a pair of Geiger counters in an anti-coincidence circuit to avoid counting secondary ray showers.

Homi J. Bhabha derived an expression for the probability of scattering positrons by electrons, a process now known as Bhabha scattering. His classic paper, jointly with 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.

[edit]Researching the particle energy

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 [ G. Clark et al.,Phys. Rev., 122:637 (1961)]. 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 all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 10²⁰ eV. 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, led by James Cronin, 1980Nobel Prize in Physics of the University of Chicago and Alan Watson of the University of Leeds. 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, due to a theoretical Greisen–Zatsepin–Kuzmin limit to the energies of cosmic rays from long distances (about 160 million light years) which occurs above 10²⁰ eV because of interactions with the remnant photons from the big bang origin of the universe.

In November, 2007 preliminary results were announced showing direction of origination of the 27 highest energy events were strongly correlated with the locations of active galactic nuclei [AGN], where bare protons are believed to be accelerated by strong magnetic fields associated with the large black holes at the AGN centers to energies of 10²⁰ eV and higher.

High-energy gamma rays (>50 MeV photons) were finally discovered in the primary cosmic radiation by an MIT experiment carried on the OSO-3 satellite in 1967 [W. Kraushaar et al., Ap. J., 177:341 (1972)]. Components of both galactic and extra-galactic origins were separately identified at intensities much less than 1% of the primary charged particles. Since then, numerous satellite gamma-ray observatories have mapped the gamma-ray sky. 

The most recent is the Fermi Observatory, which has produced a map showing a narrow band of gamma ray intensity produced in discrete and diffuse sources in our galaxy, and numerous point-like extra-galactic sources distributed over the celestial sphere.

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