THE STORY OF COSMIC RAYS — SWANN 255 



it may hurl that electron out with an energy very high compared 

 with the normal energy given to electrons by mesotrons in ordinary 

 ionization. Electrons shot out of atoms with high energy in this man- 

 ner are referred to as knock-on electrons, and they contribute appre- 

 ciably to the electron component of the radiation measured, particu- 

 larly at low altitudes. 



Neutrons. — It is of importance to observe that disintegration of the 

 protons in a primary incident atom leaves free neutrons, which should 

 therefore be fairly plentiful in the places where proton disintegration 

 occurs. Experiment, indeed, reveals the fact that neutrons do exist 

 in the atmosphere, and their intensity mounts very rapidly as high 

 altitudes are approached. It is not at present known whether the neu- 

 trons in high-energy atoms disintegrate on collision as protons do, but 

 even if such is the case, we may expect to find some neutrons that have 

 escaped destruction of this kind by becoming reduced in energy in the 

 collision process to the point at which they no longer invite destruction 

 by such a process. 



Extensive showers. — The phenomena above are the main contri- 

 butions to what we measure as the cosmic radiation. However, there 

 are other comparatively rare but very interesting phenomena. In 

 extensive showers, some of the rays observed at sea level arrive so 

 close together in time, in spite of wide separation in their places of 

 arrival, as to suggest that they have a common origin, probably in 

 one very high-energy primary particle. P. Auger, who has studied 

 these matters intensively, states that such showers of rays may corre- 

 spond to more than a million particles arriving simultaneously over 

 an area of 25 acres. If we should imagine these particles to originate 

 through pair production from a single high-energy electron, it would 

 be necessary to assume for that electron an energy between 10 15 and 

 10 16 electron volts. Taking all losses into consideration, as high as 

 10 18 electron volts for the parent particle has been estimated as neces- 

 sary to account for some of the phenomena observed. 



Nuclear disintegration caused by cosmic rays. — Another matter of 

 great interest in nuclear physics, although possibly of subsidiary 

 interest to cosmic-ray students, is nuclear disintegration produced by 

 the rays. The neutrons in the upper atmosphere doubtless play a 

 significant role in promoting such nuclear disintegrations. 



It has been customary to classify the events observed according 

 to the number of prongs which emanate from the centers of collision 

 of the rays with the nuclei of the atmospheric atoms, as seen in sensi- 

 tive films. Study of these tracks, their angular spread, their ranges 

 in matter, combined with the fundamental principles of the conserva- 

 tion of energy and momentum during collision, serve to enhance our 

 knowledge in that most mysterious realm of nuclear theory that in- 

 volves the nature of nuclear forces. 



