240 ANNUAL. KEPORT SMITHSONIAN INSTITUTION, 1935 



Later study and careful measurement only tended to strengthen 

 the interpretation that a fundamental particle of a new kind was 

 present among the cosmic-ray particles. After several more photo- 

 graphs showing evidence of a similar kind were obtained, the dis- 

 covery was announced early in September 1932. 



In March 1933 P. M. S. Blackett and G. P. S. Occhialini in Cam- 

 bridge, England, confirmed our announcement of the existence of 

 the positive electron or positron. They used an apparatus of a 

 similar kind, though with the added advantage that a cosmic-ray 

 particle in passing through their apparatus would itself actuate the 

 mechanism and cause a photograph of its path to be taken. 



During the following weeks workers in California and in various 

 European laboratories carried out lengthy investigations on posi- 

 trons. It was found that it was not necessary to rely upon cosmic 

 rays, but that rays of laboratory origin could produce them as well. 

 First, Chadwick, Blackett, and Occhialini showed that positrons 

 were produced by the radiations generated when alpha particles 

 from a radioactive substance were allowed to strike beiyllium. The 

 radiations in this case are complex in character, consisting both 

 of neutrons and gamma rays; but in their experiment it was not 

 possible to determine which of the rays was responsible for the pro- 

 duction of the positrons. Curie and Joliot by a similar experiment, 

 in which they inserted blocks of lead and paraffin into the path of 

 the rays producing the positrons, showed that the positrons arose 

 more likely as a result of the gamma rays than of the neutrons. 



The first direct proof that the well-known gamma rays from tho- 

 rium can give rise to positrons was given by S. H. Neddermeyer and 

 the writer. If the gamma rays are allowed to pass through a rather 

 thick piece of lead and if the ejected electrons are observed, most of 

 them are found to be of negative sign although some 10 percent are 

 positive. Besides determining the relative numbers of positive and 

 negative electrons, these experiments determined also the speeds or 

 energies with which the electrons of both signs were ejected. 



Here an observation was made which is of importance in deciding 

 just how the positrons are produced. The most energetic positrons 

 were found to have less energy than the most energetic negative 

 electrons by a definite amount corresponding approximately to 1,000,- 

 000 electron-volts.^ 



To point out the significance of the energy difference of 1,000,000 

 electron-volts, we must go back a few years and consider what the 

 mathematical physicists had been thinking and doing while the other 

 class of physicists, the experimenters, were performing their tests. 



* The electron volt is a convenient unit for the measurement of electron energies and Is 

 equivalent to 1.6 XIO'^ ergs or 1.2xl0-« foot-pounds. 



