THE STORY OF COSMIC RAYS — SWANN 249 



in this matter are almost exclusively electrons, for although, in prin- 

 ciple, heavier particles can operate in an analogous manner, the effect 

 is so much more prominent in the case of the lighter electron that we 

 can ignore it in the other cases. 



Start with a high-energy photon which finds itself directed toward 

 an atom. This photon possesses the characteristic of becoming mathe- 

 matically irritated when it comes into the vicinity of the atom. Exist- 

 ence as a photon becomes mathematically intolerable, but nature has 

 provided for it the option of changing its state of existence by allow- 

 ing it to materialize into two oppositely charged electrons ; these share 

 the energy of the photon between them, but not necessarily in equal 

 amount. 



Each of the pair of electrons thus produced pursues its course, and 

 if either has sufficient energy, when entering the domain of another 

 atom it will jerk that atom in such a manner as to give rise to a new 

 photon. This photon, if of sufficient energy, will repeat the history 

 of its ancestors, giving rise to two more charged particles. The process 

 would go on and on were it not for the fact that each of these progeny 

 has only a small share of the energy of the original photon ; and when 

 the energy of a particle falls below about 10 million electron volts, 

 the chance of its reproducing itself in this manner becomes infinitesi- 

 mal. (An electron volt is the energy gained by an electron in falling 

 through a drop of potential of one volt.) 



Once the energy of an electron has fallen below the value necessary 

 to carry on the process of photon emission and pair production, its 

 remaining energy is gradually drained away from it by ionization and 

 it loses the characteristics of a high-energy ray. 



In the light of the foregoing, it may be expected that if a high- 

 energy photon or electron enters our atmosphere, or comes into exist- 

 ence in the upper atmosphere as a result of the primary cosmic rays, 

 this electron or photon will initiate the phenomenon of pair produc- 

 tion. As we descend into the atmosphere, the number of electrons 

 passing through a unit area will at first increase, attaining finally a 

 maximum, after which it will decrease. This is because there is a 

 birth rate of electrons resulting from the pair production and a death 

 rate resulting from electrons falling to an energy at which they are no 

 longer able to perpetuate the process. Highest in the atmosphere is 

 a region where the birth rate exceeds the death rate ; below that the 

 two are equal and the number of rays is a maximum ; further descent 

 takes us to regions where the death rate exceeds the birth rate and 

 the number of electrons diminishes. 



As we proceed in the study of cosmic rays, we shall find that pair 

 production plays a significant role in the phenomena observed. 



Properties of mesotrons. — The need for the existence of a charged 

 particle intermediate in mass between the electron and the proton was 



