PHOTONS AND ELECTRONS 41 



of the energy given by the relation in question. This prediction also is 

 verified (sometimes by using the Wilson chamber, observing the electron 

 tracks which start off in various directions from atoms of the gas in the 

 chamber, estimating the corresponding electron energies and correlating 

 them with the corresponding values of 4>). 



In general, when a beam of X-rays or gamma rays falls upon a piece 

 of matter, some of the electrons which fly out are "Compton electrons" 

 which acquire their energy in the foregoing matter, and some are photo- 

 electrons which have been ejected from atoms by total absorption of 

 X-ray quanta such as I described in a previous section. Compton 

 electrons are relatively more abundant the higher the frequency of the 

 X-rays and the lower the atomic number or atomic weight of the scatter- 

 ing element, i.e., the higher the ratio of the energy of the photons to the 

 energy required to extract electrons from the atoms. Also, when a beam 

 of X-rays or gamma rays falls upon a piece of matter, some of the photons 

 which emerge are the photons of reduced frequency recoiling from 

 impacts with electrons, of which we have just been thinking; some are 

 photons scattered without change of frequency; some are X-rays emitted 

 from atoms as an aftereffect of the extractions of electrons by incident 

 photons. The reduction in intensity which a beam of X-rays or gamma 

 rays suffers in traversing a stratum of matter is largely due to these three 

 processes jointly; the two former, as compared with the last-named, are 

 relatively more prominent the higher the frequency of the incident 

 rays and the lower the atomic weight of the scattering substance. With 

 gamma rays of great energy, a novel process enters upon the scene. 



TRANSMUTATION BETWEEN ELECTRON-PAIRS AND PHOTONS 



It is fitting to end this essay by a brief allusion to one of the most 

 striking phenomena of Nature, unguessed as lately as three years ago. 

 In addition to the negative electrons of which so much has just been 

 said, there are positive electrons, first observed in 1932 among the cosmic 

 rays. These are apparently identical with those of the negative variety 

 in every respect except the sign of their charge. They are observed 

 immediately as they come into being, for they may be created in the 

 laboratory by the transmutation of light into electricity. When a beam 

 of gamma rays (the photons must have energy superior to about one 

 MEv) is projected against a stratum of matter, pairs of electrons of opposite 

 sign spring forth. Each pair is formed out of a photon. The mass 

 and the energy of the photon are converted almost totally into mass 

 and energy of the two electrons, a trifling fraction being transferred to 

 an atom in order to satisfy the law of the conservation of momentum. 

 The reverse process occurs when positive electrons are fired into a stratum 



