CONTEMPORARY ADVANCES IN PHYSICS 205 



When finally the extension was made by people interested in the 

 cosmic rays, it turned out that according to the quantal theory the 

 liability of electrons to these "radiative energy-losses" goes up so 

 greatly with increasing speed, that electrons of even the cosmic-ray 

 energies should not be able to bore their way through as much as five 

 centimetres of lead ! 



After the meaning of this inference sank in, there ensued a period 

 lasting for months (in 1935 and 1936) in which several eminent theorists 

 were willing to concede that Nature must have set a limit to the 

 scope of quantal theory. It was beginning to be believed that some- 

 where between the energy-range attainable in the laboratory and the 

 energy-range manifest in the cosmic rays, there is a critical energy- 

 value beyond which the electron escapes from the sway of the quantal 

 laws, and is exempted from losing its energy by the process of Brems- 

 strahlung. This belief was an artifice for permitting the penetrative 

 particles of the cosmic rays to be called by the name of electron. 

 It might have remained a credible artifice, if the penetrative particles 

 had been the only ones — if, that is to say, there had never been any 

 evidence for the existence of particles among the cosmic rays having 

 the properties required of electrons by the quantal theory. Such a 

 situation may have seemed to exist at the time when the belief was 

 dominant. It exists no longer, as the description of Fig. 8 has just 

 suggested; but before considering further the data, I must introduce 

 something more of what the theory has to say. 



Since 1934 it has been known that a photon of energy greater than 

 about one million electron-volts is capable, when in the vicinity of an 

 atom-nucleus, of converting itself into a pair of electrons of opposite 

 sign. About one million electron-volts — 1.02 Mev, to be somewhat 

 more precise — becomes "rest-energy" of the twin electrons, being 

 incorporated with their rest-masses; the remainder (hv — 1.02, if by 

 hp we denote the photon-energy in Mev) becomes kinetic energy of 

 the electrons. The process may be produced at command and 

 exhibited to the eye, by projecting the photons known as gamma-rays 

 against metal targets contained in expansion-chambers. The gamma- 

 rays originally used for this purpose proceeded from natural radio- 

 active substances; mostly they were those emitted by a certain sub- 

 stance (thorium C") with a photon-energy of 2.62 Mev. Nowadays 

 gamma-rays of energy several times as great can be produced by 

 effecting certain transmutations, in the course of which (or afterward) 

 they emerge from the new-born nuclei. Figure 10 shows an admirable 

 example of an electron-pair formed out of such a photon. Moreover, 

 the converse process is well-known: positive electrons falling against 



