298 BELL SYSTEM TECHNICAL JOURNAL 



yet been related in the pages of this journal, and as they are extremely 

 interesting portions of the most strictly contemporary physics, they 

 well deserve some pages of description. 



The Neutron "^ 



It had been known since 1919 that certain light elements emit 

 protons when they are bombarded by alpha-particles; these, however, 

 are not "penetrating" rays, in the sense in which that term is 

 commonly used, inasmuch as they are completely stopped by a layer 

 of metal a fraction of a millimetre thick. The discovery of the neutron 

 was the outcome of an attempt to detect penetrating rays emitted by 

 the bombarded atoms. Bothe and H. Becker made this attempt, 

 surrounding the source of alpha-particles and the substance on which 

 they impinged by two millimetres of zinc and brass, and detecting 

 what got through this barrier by means of a Geiger point-counter. 

 Four elements — lithium, boron, fluorine and especially beryllium^ 

 produced an unmistakable effect. Bothe and Becker ascribed this to 

 high-frequency gamma-rays or photons. It was indeed largely due 

 to such photons; but mingled with these there were particles of another 

 nature, as the further experiments of Irene Curie, Joliot and Chadwick 

 were to prove. 



To appreciate the proof it is necessary to realize that what is ob- 

 served is an indirect rather than the direct effect of the corpuscles 

 coming from the atoms bombarded by the alpha-rays. It is ionization 

 of gas which is observed — ionization coming in spurts, which may be 

 separately observed and counted by use of a Geiger counter or a quick- 

 acting electroscope with proper amplifiers or an expansion-chamber, or 

 may be summed up by the accumulation of charge in a slow-acting 

 electrometer. The spurts of ionization are due to the transits of 

 corpuscles across the gas, corpuscles which sometimes at least are 

 recognizably electrons or atom-nuclei. But it is not to be taken for 

 granted that these directly-ionizing corpuscles spring from the source 

 of the phenomena, the element bombarded by the alpha-particles. 

 They start their flights in the matter environing the source, being 

 launched on their courses by invisible agents which are presumably 

 the true primary rays coming from the source. What is observed, 

 therefore, depends on the matter surrounding the source; and the last 

 step leading up to the identification of the neutron was taken when 

 Curie and Joliot interposed thin screens of various substances in the 

 path of the primary rays from the source to the ionization-chamber. 



2 F"or a fuller account cf . an article of mine in Review of Scientific Instruments, 4, 

 58-63 (February, 1933). 



