Cosmic Ray
Cosmic rays are a component of interstellar matter. Their composition is similar to ordinary interstellar gas, but they are hurtling through space at very high speeds.
In 1911, physicist Victor Franz Hess made balloon ascensions with an electroscope to measure background radiation that seemed to be emanating from everywhere on the Earth. To his surprise, the higher he went, the more radiation he detected.
In an attempt to discern the source of the radiation, Robert A. Millikan and his associate George H. Cameron (1902-1977) lowered an electroscope into a lake in 1925 and detected radiation more powerful than any previously known emission. By taking measurements at different locations, they concluded that the source of the radiation was outer space, and Millikan coined the term cosmic rays. Determining what they were, however, was another matter.
Two theories were forwarded; Millikan suggested that cosmic rays were high-energy photons, created as a result of a nuclear charge. Another theory suggested they were charged particles. Arthur Holly Compton discovered the true nature of the rays in 1932. He organized a survey of the entire globe and noticed that there was a difference in the detection of cosmic rays based on latitude. At higher latitudes, more particles were detected. Since magnetic fields cause charged particles to deflect, it was fairly certain that the Earth's magnetic field was influencing the rays, deflecting them toward the poles. The rays had to be charged particles.
Millikan had been working with Carl David Anderson to determine the nature of cosmic rays. Anderson's work led him to the discovery of the positron and the development of nuclear physics.
In 1958 a team of scientists, led by James Van Allen, studied data gathered by the Explorer I satellite. It had discovered an entire belt of charged particles surrounding the Earth like a doughnut. These Van Allen belts are composed of trapped cosmic rays that spiral down to the poles.
Most of the cosmic rays are hydrogen nuclei that have lost their accompanying electron. They travel at very high speeds, typically at speeds equal to 0.9 times the speed of light. But where do these particles originate? That is difficult to determine; by the time the Earth's magnetic field has finished twisting a particle around, it is impossible to trace it back to its source. The most likely source for cosmic rays appears to be a supernova explosion. This is a violent death that affects massive stars. Tremendous amounts of energy are created in a supernova, and there are enough explosions going on to account for the number of cosmic rays that have been detected. How the energy in an explosion manages to accelerate the protons and other atomic nuclei to cosmic ray level is yet to be understood.
Supernova explosions are known to occur in other galaxies as well. Some of the cosmic rays we detect must have originated from outside our Milky Way, although the magnetic field of our galaxy might deflect all but those with the highest energy.
Another source is the Sun, which produces weak cosmic rays. The Sun emits a solar wind composed of low-energy charged particles. When the charged particles encounter our magnetic field, they spiral along the magnetic lines to the north and south poles. As they spiral down, they may cause our atmosphere to glow, producing the beautiful aurora borealis and aurora australis, the northern and southern lights.
The Sun also blocks cosmic rays. When it is in an active cycle, its magnetic field increases and deflects cosmic rays from deep space away from the Earth. On the other hand, during the active period the production of its own cosmic rays increases.
In the 1990s, a huge project was begun to study the so-called "super cosmic rays," or SCRs. These are the most energetic cosmic rays, and their origin is a mystery because there are few known processes that can produce particles with the enormous energy possessed by the SCRs. The Pierre Auger (pronounced aw-ZHAY) Observatory is an international project, under development as of 1998. It will consist of two arrays of SCR detectors, with the elements of each array spanning an area of 3,000 square kilometers. The detectors are water tanks capable of detecting Cherenkov radiation, a type of radiation produced by the SCRs, and there will be about 3,200 of them! When fully operational, the Auger Observatory will allow us to probe the nature of some of themost energetic particles in the universe.
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