COSMIC RADIATION — JOHNSON 211 



types of spontaneous nuclear disintegrations. Though considerably 

 more rare than their negative counterpart, they could possibly be 

 present in the cosmic radiation. The second possibility is the posi- 

 tive proton, the nucleus of the more common form of hydrogen. 

 These occur in large numbers on stars and in the interstellar regions 

 and are very likely candidates. This possibility would also include 

 the mass 2 and the mass 3 hydrogen nuclei which occur in certain 

 small proportions with the ordinary hydrogen. The third possi- 

 bility is the alpha particle or helium nucleus, which also occurs abun- 

 dantly throughout the universe. Other heavier nuclei might also 

 be included in this class. Their multiple charge^ and the consequent 

 rapid loss of energy in traversing matter would seem to put these 

 particles out of the picture as far as the sea-level intensity is con- 

 cerned, though they might well contribute to the intensity in the 

 upper atmosphere. In fact alpha particles have been proposed by 

 Compton as an explanation of some of the radiation observed in the 

 stratosphere. At sea level and up to the tops of mountains, the 

 principle candidates for the cosmic rays are thus the proton and 

 the positive electron. 



If the rays are protons, the component observed at the Equator in 

 the asymmetrical band at 30° from the vertical, lies in the range of 

 energies from 11 to 20 billion volts. If the rays are positive electrons 

 the energies extend from 12 to 21 billion volts. The range is outside 

 the limits of previous experience, and it is necessary to rely upon the 

 predictions of untested theories for the final identification of these 

 rays from their absorption characteristics. The test is difficult be- 

 cause the theories show on quite general grounds that rays of equal 

 charges but different masses behave nearly alike if the energy is 

 large compared with the energj^ equivalent of the masses. The mass 

 of the proton is equivalent to 1 billion electron volts and the lighter 

 electron is equivalent to a half million electron volts. Both are 

 small compared with cosmic ray energies. Theoretically the two 

 kinds of rays should be absorbed by matter nearly alike. For the 

 process of the excitation of photon rays by collisions with nuclei, 

 however, the difference in mass may be significant, and there are 

 reasons for expecting the electron with its smaller mass to excite 

 photons more readily than the proton. To the extent that radiation 

 losses are important ways for rays to lose energy, the protons should 

 be the more penetrating. 



At this stage in the analysis it would be extremely helpful to find 

 a method of selecting one type of ray to the exclusion of the other, 

 and if photon excitation is a unique characteristic of electron rays, 

 an arrangement of apparatus, sensitive only to photons, would ac- 

 complish the desired end. Kecent studies of the shower phenomena. 



:!6923— 36 15 



