238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



The scene now shifts to France, and we find F. Joliot and his wife, 

 Irene Curie, the daughter of the Late Madame Curie, studying further 

 the new Bothe-Becker rays. They brought to light the very im- 

 portant fact that when these rays pass through matter they impart 

 energy to nuclei and set them in rapid motion. These rays are then 

 unlike ordinary gamma rays, which are known to impart energy 

 almost wholly to electrons only. But still the rays kept their secret; 

 and again the scene shifts, this time to England. 



J. Chadwick, profiting by the German and French experiments, 

 turned his attention to these mysterious rays. He measured the dis- 

 tance that various kinds of nuclei set into motion by the rays were 

 able to penetrate through matter. In a short time he realized that 

 the gamma-ray interpretation was entirely inadequate; but that all 

 the effects could be coherentlj^ explained by assuming that the rays 

 consist of a stream of particles about as massive as protons, although 

 unlike protons, and unlike any material particles ever before ob- 

 served, they are neutral electrically. Despite the initial skepticism 

 on the part of many scientists, in a few weeks' time, after further 

 work by Chadwick and by others, this very revolutionary interpre- 

 tation of the experiment was universally accepted and the existence 

 of the neutron was established. The neutron was almost, but not 

 entirely, a new concept, for in 1920 both Harkins and Rutherford 

 had suggested the possible existence of such a particle, although not 

 until 1932 were the experiments performed and the proof obtained. 

 That date, therefore, remains as the neutron's birthday. 



The discovery of the neutron was not by any means unwelcome, for 

 it was immediately recognized that instead of electrons and protons 

 one might use neutrons and protons out of which to build nuclei, 

 thus avoiding the difficulty previously mentioned of confining elec- 

 trons within nuclear dimensions. In fact, Heisenberg announced 

 a preliminary theory of nuclear structure based on such a model. 



Chadwick suggested that the neutron itself may represent a very 

 close combination of electron and proton ; though it is not impossible 

 that it may be itself a new fundamental particle. 



THE POSITRON 



During the time that the European physicists were busying them- 

 selves with the experiments which led to the discovery of the neutron, 

 in California there was in progress another tjqoe of experiment which 

 resulted in the discovery of the positive electron or positron, as this 

 particle is now generally designated. 



The subject of study in this instance was the cosmic radiation. In 

 the spring of 1930 Professor Millikan and the writer, in collabora- 

 tion, planned an apparatus designed to measure directly the sx^eeds 



