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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 10 — An electron-pair born from a photon. (VV. A. Fowler, California Institute 



of Technology) 



a plate of dense matter bring about the emission of photons of energy 

 0.51 Mev, and these are just what are to be expected if the positive 

 electrons (after being slowed down) unite with some of the innumerable 

 negative electrons already in the plate and produce, at every such 

 union, a pair of equal photons.* Much too abundant to be here 

 described is the evidence for the ability of electron-pairs to pass into 

 light and light to pass into electron-pairs, making it permissible to 

 imagine a continual alternation of energy between these two so sharply 

 contrasted forms. 



Formation of the photons of Bremsstrahlung by electrons of enormous 

 energy, and formation of electron-pairs out of such photons: these 

 reciprocal processes engaged the attention of several theorists (Bethe, 

 Heitler, Sauter, Weiszaecker, Oppenheimer) in the years 1933 and 

 1934. The problem was, to evaluate by quantal theory the chance 

 that electron or photon would spend its energy in producing photon 

 or electron-pair, while traversing given thickness of given element. 



* Evidently this is not quite the converse of the process previously described, 

 which if reversed would consist in the merger of a positive and a negative electron 

 with the formation of a single photon bearing away all of their energy. Some 

 evidence exists for the occurrence of this process. There is no sign of the fourth 

 conceivable process (the meeting and merger of two photons to form two electrons) 

 which must obviously be very rare in practice owing to the feeble concentration of 

 photons in actual beams of gamma-rays. Nevertheless this last is the process first 

 predicted by the theorist Dirac. 



