300 BELL SYSTEM TECHNICAL JOURNAL 



The argument is as follows: For simplicity let us consider solely the 

 nuclei which are projected in directions pointing straight away from 

 the source of the primary rays, and therefore must have suffered 

 central impacts. Specially, let us take the cases of hydrogen and 

 nitrogen nuclei thus projected. The ranges of these have been 

 measured (of N by Feather, of H by various physicists) and their 

 maximum speeds deduced by means of knowledge earlier acquired of 

 the range- vs. -speed relations of charged particles. The values of speed 

 accepted by Chadwick are 3.3-10^ and 4.72-10^, respectively. Now 

 if the corpuscles which in central impacts gave to these nuclei these 

 speeds were photons, it is easy to compute by the Compton-effect 

 equations the energy U of the photons; if the impinging corpuscles 

 were material particles of mass M and speed v, it is easy to compute 

 both V and M. It turns out that by the first procedure, one gets 

 different values of U from the two cases (55 and 90 million electron- 

 volts, respectively); by the second, one gets compatible values of 

 M and v. With the first theory, then, one would have to say that 

 nuclei of different kinds were struck by different photons. This is 

 not quite inconceivable, as there might be a mixture of gamma-rays 

 of different energies, and a greater likelihood of the higher-energy 

 photons interacting with the more massive nuclei. But it seems less 

 acceptable than the other theory, which permits one to postulate a 

 single kind of corpuscle to explain the impacts against both kinds of 

 nucleus. This corpuscle must be neutral, as a particle of charge e and 

 the computed mass and speed could never penetrate nearly as thick a 

 layer of matter as it can traverse; it is therefore called the "neutron." 



The value of M deduced from the foregoing data is given as 1.15 

 times that of the hydrogen nucleus; the possible error in the estimate 

 of the speed of the recoiling nitrogen nuclei is such that Chadwick 

 says "it is legitimate to conclude that the mass of the neutron is very 

 nearly the same as the mass of the proton." An estimate ostensibly 

 much closer (1.007 ± .005) has been made by a train of reasoning 

 which I will later quote. 



The Positive Electron 



Whereas the discovery of the neutron came about through the study 

 of transmutation, the positive electron came to light in the course of 

 cosmic-ray research. The ionization of the atmosphere, whereby the 

 cosmic rays are manifested, is due directly to fast-flying corpuscles 

 which leave behind them trails of ionized molecules fairly close to- 

 gether (on the average, about a hundred ion-pairs per cm. in air at 

 sea-level atmospheric pressure). The trails may be made visible by 



