Unit 20 Grammar: Elliptic Sentences
Text 20 Л COSMIC RAYS
Primary cosmic rays are submicroscopic particles that travel in space outside the earth's atmosphere at speeds nearly equal to that of light. Some of them happen to approach the earth and enter the atmosphere.
High in the atmosphere, most of the primary cosmic rays collide violently with the atoms they encounter in the air, in which case they impart their energies to the fragments resulting from the collision. In effect, these fragments or secondary rays are what we observe at lower levels. Like the primaries they too collide with atoms in the air, or eventually with atoms in the earth, until ultimately the energy is all transformed into heat. The term cosmic rays is used to refer both to the primary and secondary rays.
Although an ancient phenomenon, cosmic rays because of their small effects went unrecognized until the end of the last century and the beginning of the present century. The total energy of all cosmic rays arriving in the atmosphere per unit time is only about 10 microwatts per square meter, roughly equal to the energy in starlight and a 100,000,000 times less than the radiant energy from the sun. Therefore, cosmic rays do not affect life on the earth appreciably in any direct, physical way.
The discovery of the existence of cosmic radiation was a consequence of certain experiments undertaken on the conductivity of gases. It was believed, on theoretical grounds, that a gas should be non-conducting in the absence of radiation, provided that the potential gradient across it was not so high that sparking could take place. Curiously enough, experiments undertaken to test this hypothesis showed that a sample of air in a closed vessel always exhibited a small electrical conductivity in spite of every precaution to eliminate radiation, and prevent leakage along the insulators. The conductivity was observed to increase in proportion to the pressure of the enclosed air, and to be diminished by surrounding the vessel with thick shields; therefore, it seemed to be due to some kind of radiation continually entering the vessel through the walls. If so, this was a more penetrating radiation than had ever been known before.
Various suggestions were advanced to explain this phenomenon, among them residual radioactivity of the shielding materials, and spontaneous ionization due to the thermal motion of the gas molecules.
That these explanations were not sufficient to account for the observed phenomena was shown by the experiments of some scientists who, in the years immediately prior to 1914, sent ionization chambers up with balloons, and measured the variation of the conductivity of the contained gas as a function of altitude. They were able to show that conductivity, and hence the ionization produced in the gas, somewhat decreased up to an altitude of about 2,000 feet above sea-level, indicating sources on the earth, and thereafter increased steadily up to the highest altitude which their balloons reached (30,000 feet), at which altitude it was many times greatei than at sea-level. From this experimental result it was clear that, whatever the source, the whole of the residual ionization observed at sea-level could not be attributed to the radioactivity of the earth, nor can it be a property of the gas with which the ionization chamber is filled. Tha* the immediate source of the radiation is not the sun is a consequence of the fact that ionizatio? was the same, whether day or night, and was therefore not due to rays coming directh from the sun.
Although the above conclusions were confirmed by a number of physicists in the yean immediately prior to the outbreak of the first world war, it was not until 1926 that the existenc< of the cosmic radiation was generally accepted.
The distinctive feature of cosmic rays is a unique concentration of energy in single elementary particles. Though apparently similar in substance (mainly protons or nuclei of hydrogen) to the rays that cause aurora borealis, primary cosmic rays have individual energies about a million times greater, and penetrate far into the atmosphere and occasionally deep into the earth. Whereas the average quantum energy in starlight is merely 2 ev, the average energy of single particles in the primary cosmic radiation is 1010 times greater, or about 20 BeV.
Such particles are not deflected by the electric forces that normally keep atoms and particularly the nuclei of atoms apart; they can penetrate through the middle of any nucleus and cause it to disintegrate and they can create out of their kinetic energies new types of unstable particles which otherwise would not exist naturally on the earth. The cosmic rays do not obey Newton's laws of mechanics as do slower-moving bodies, but provide extreme examples requiring application of the principles of relativity.