242 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 19 3 5 



positron, then the calculated energy required for the creation of a 

 pair of electrons comes out very close to 1,000,000 electron volts. 

 The agreement with experiment is good, for according to this picture 

 a negative electron can receive practically all the energy of the inci- 

 dent radiation ; whereas a positron can appear only through the crea- 

 tion of a pair, hence the maximum energy it can receive is the energy 

 of the incident radiation diminished by the 1,000,000 electron volts 

 required to create the pair. 



In plate 2, figure 1, is shown a photograph of the paths of a pair 

 of electrons, one positive and one negative, generated by gamma rays 

 from thorium. Repeated experiments by ourselves, including meas- 

 urements on 2,500 electrons, and by Chadwick, Blackett, and Occhia- 

 lini, based on 4,000 measurements, have failed to show any certain 

 evidence that the positrons are not created along with negative 

 electrons by the incident gamma radiation. 



Calculations made by J. R. Oppenheimer and M. S. Plesset, who 

 used the Dirac theory, have shown that the attenuation of a gamma- 

 ray beam in passing through matter (due to the energy spent in 

 creating electron pairs) is in good accord with observations. 



This bold theory of Dirac requires further that a positron, w^hen 

 it finds itself in a very ordinary environment, as, for example, in 

 passing through water, shall, on the average, have only a very short 

 life, of the order of a billionth of a second or less ; for when a posi- 

 tron meets a negative electron, both particles will suffer the fate of 

 complete annihilation, and in their stead a pair of corpuscles ot 

 radiant energy, each of one-half million electron volts, will remain. 

 Although the lifetime of positrons has not been actually measured, 

 it has been shown to be very short, and the " annihilation radiation " 

 announcing their death has been observed. The first to do this were 

 Joliot and Thibaud. The annihilation radiation is of the proper 

 intensity and the energy of its individual corpuscles is approximately 

 the required amount of one-half million electron volts, corresponding 

 to the complete annihilation of the positrons. There is no reason to 

 believe, however, that a positron, if removed from a region densely 

 populated by negative electrons, may not live hundreds of millions 

 of years, instead of perishing in a billionth of a second. 



The exact relationships among the four primary particles, the 

 negative electron, the positron, the proton, and the neutron, are at 

 present not known. It appears quite plausible that under certain 

 conditions a negative electron and a proton may be formed out of 

 a neutron; and similarly a positron and a neutron may be formed 

 out of a proton. 



A photograph showing the paths of a shower of positrons and 

 negative electrons ejected from a plate of lead by cosmic rays is 

 presented in plate 1. 



