THE STORY OF COSMIC RAYS — SWANN 253 



can be bent by the earth's magnetic field, neutrons could not reach the 

 side of the earth opposite their point of origin. 



The production of mesotrons. — When the protons of the primary 

 radiation enter our atmosphere, they very rapidly disintegrate into 

 mesotrons, as the result of collisions with the nuclei of atoms. The 

 protons disappear so rapidly by this process that only about a third of 

 them remain at an altitude where the pressure is y 10 of an atmosphere. 



Even in the case of the heavier atoms contained in cosmic rays, it 

 is the protons in their nuclei that are responsible for giving birth to 

 mesotrons ; and since mesotrons are the most important constituent of 

 the rays that we observe at lower altitudes, a helium atom, containing 

 two protons, is twice as effective in producing mesotrons as is a single 

 proton. An atom like iron, with 26 protons in its nucleus, is 26 times 

 as effective as is the proton itself. If iron atoms, for example, were 

 only 1 percent as numerous as free protons, they would nevertheless 

 contribute about one-third as many mesotrons as do the free protons. 

 Thus, despite the relatively small percentage of atoms of high atomic 

 weight, it turns out that 50 percent of the mesotrons produced in the 

 atmosphere come from primaries that are heavier than protons. 



When a proton, either free or in combination in a nucleus, enters the 

 atmosphere and collides with the nucleus of an atom of air, it is prob- 

 able that, in the first instance, pi-mesotrons are produced, as shown 

 in the genealogical chart on page 251. A slowly moving pi-mesotron 

 with a mean life of 10 -8 second would travel only a meter or so before 

 disintegrating to form a mu-mesotron. A slow mu-mesotron, with 

 a mean life of the order of 2.3 microseconds, would go less than 700 

 meters before disintegrating into an electron. 



Mesotrons of higher energy live longer and penetrate farther, 

 so that a mesotron of 10 billion electron volts would live for about 

 %oooo °f a second and in that time could travel 60 kilometers. Thus, 

 from mean-life considerations alone it would have no difficulty in pene- 

 trating the whole atmosphere. As a matter of fact, only about 1.3 

 billion volts of its energy are necessary to allow for all the ionization 

 it would cause on such a journey, so that neither ionization loss of 

 energy nor mean-life considerations would prevent such a mesotron 

 from reaching the earth's surface. Indeed, some of the mesotrons 

 are produced with such high energies that they can penetrate far 

 more than the thickness of the earth's atmosphere, and cosmic-ray 

 intensity has a measurable value at depths below the earth's surface 

 comparable with 250 meters of water. 



Starting in the outer regions of the atmosphere, we have only the 

 primaries. These decrease rapidly with descent, by the production of 

 mesotrons, so that the mesotron intensity rises as we descend into the 

 atmosphere until, at a depth of about y 10 of the whole atmosphere, 



