CONTEMPORARY ADVANCES IN PHYSICS 



103 



but these are not a proper standard of comparison. Rather should 

 one say that in the autumn of 1932 positive electrons were being 

 observed at the rate of three or four a year, and already by the summer 

 of 1933 this rate had been enhanced to thirty thousand in the second! 

 The other voluntary way of generating positive electrons — -by 

 applying hard gamma-rays to heavy elements — ^has already been 

 studied enough to yield the data of the following table. Here, in the 

 first column, stand the names of various sources of gamma-rays (the 

 one denoted as "Po + Be" is beryllium exposed to impacts of alpha- 

 particles from polonium); in the second, the energy-values in MEV 

 (I use this symbol hereafter for "millions of electron-volts") of the 

 individual photons of these rays; in the third, the symbols of various 

 metals; in the fourth, the number of positive electrons per hundred 

 negatives, ejected from these metals by these gamma-rays; in the 

 fifth, the authorities: 



The percentages in the fourth column give at the moment our best 

 available notion as to the relative plentifulness of positive electrons, 

 produced by the several kinds of rays falling upon the several metals. 

 One would prefer to have the total number of positives per unit 

 intensity of the infalling rays, but that is not available at present — - 

 I presume because of the difficulty of measuring these intensities. 

 One must remember that the data usually consist in observations of a 

 few hundred or a few dozen cloud-tracks, so that the accuracy of 

 these percentages cannot be great. ^ 



We note that with lead the proportion of positive electrons mounts 

 rapidly with increasing photon-energy, and that with 5 MEV-photons 



^ This perhaps is sufficient to account for a discrepancy between the general trend 

 of the table and a value of 1/3 given by Meitner and Philipp for the ratio of positives 

 to negatives when brass is exposed to (Po + Be). Should the table be extended and 

 supported by a successful theory, it should then be possible to determine the frequency 

 of gamma-rays by the percentage of positives which they produce when falling on a 

 metal. In this connection it is interesting that Anderson's latest data indicate that 

 positive and negative electrons are about equally abundant among the ionizing 

 particles of the cosmic rays, a fact which suggests that if they are due to photons, 

 these must be of a distinctly higher energy than anv of those cited in the foregoing 

 table. 



