THE STORY OF COSMIC RAYS — SWANN 267 



Death occurs to a cosmic ray when, in its wanderings through the 

 galaxy, it strikes another atomic particle and disintegrates into meso- 

 trons, and these in due course decay into electrons and neutrinos. The 

 chances of collision are small; a cosmic ray may travel for many 

 millions of years without hitting another atomic nucleus and dying. 

 However, a larger atom will collide more frequency than a smaller 

 one, and so lifetime considerations will tend to favor the lighter 

 particles. 



From the researches of astronomers, physicists, and chemists, we 

 have found the observed relative abundances to be: 



Sun, stars, 

 Element Cosmic rays interstellar matter 



H 100, 000 100, 000 



He 25, 000 10, 000 



C, N, O 900 130 



Heavier elements 10 15 



This table does not favor the light elements in relation to heavy 

 ones, although carbon, nitrogen, and oxygen appear to be relatively 

 more abundant in cosmic rays than in the universe as a whole. How- 

 ever, the mechanisms of acceleration may act more strongly on one 

 atom than on another, and there is, after all, very little reason to sup- 

 pose that the atoms of different elements have an equal chance of 

 receiving cosmic-ray energies. 



Nevertheless, one cannot doubt the importance of obtaining further 

 more definite knowledge of the relative abundances of elements in 

 the primary cosmic rays, and of the relative energies acquired by the 

 different kinds of particles. From data of this kind, combined with 

 further studies of such phenomena in the cosmos as a whole, we may 

 hope some day to understand in greater detail all the processes in- 

 volved in the life histories of these rays from the time of their creation 

 from ordinary matter to their entry into our atmosphere. 



