308 Sir E. Rutherford on the 



Distribution of energy between ft and 7 rays. 



It was initially supposed that a large fraction of the 

 ft radiation from substances like radium B and radium C, 

 which give a marked /3-ray spectrum, appeared in the form of 

 the homogeneous groups of ft rays observed. J. Chadwick * 

 has shown, however, in a recent paper that even the intense 

 lines in the magnetic spectrum of radium B represent only 

 a small fraction of the total number of ft rays emitted. 

 This result was obtained by direct counting of the ft particles 

 and confirmed by showing that an increase of intensity of 

 only a few per cent, of the ft radiation falling at a given 

 part of the photographic plate gave the impression on 

 development of a strongly marked band. 



We may conclude from these results that the magnetic 

 spectrum of the ft rays from radium B or radium C consists 

 of a continuous spectrum of ft rays on which is superimposed 

 a line spectrum corresponding to groups of rays expelled at 

 definite speeds. A satisfactory explanation of these results 

 and also of other marked differences in the distribution of ft 

 and 7 rays from radioactive substances can, I think, be given 

 on the following lines. Suppose — as seems probable — that 

 the disintegration of the atom leads to an expulsion of a high 

 speed ft particle from or near the nucleus. This ft particle 

 in passing through the outer distribution of electrons will,, 

 on the average, suffer several collisions of an ordinary type 

 with the electrons, and will share its energy "with them. As 

 a statistical result of a large number of atoms, the velocity of 

 the escaping ft particles will, on the average, be continuously 

 distributed within certain limits of velocity. This would 

 give rise to the continuous spectrum of ft rays which is most 

 typically illustrated by the ft ra} r s from radium E. Next 

 suppose that there are certain w r ell~defined regions in the 

 electronic distribution which can be set into definite vibration 

 by the escaping ft particle. These regions are to be identified 

 as containing the particular structures which give rise to the 

 " characteristic " 7 radiations from the atom. If some of 

 the ft particles in escaping from the atom pass through one 

 or more of these regions, they give rise to a line-spectrnm 

 of 7 rays and at the same time to one or more groups of 

 ft rays of definite speed. The connexion between the energy 

 of the 7 ray and of the ft ray will be discussed later. 



On this view, the appearance of homogeneous groups of 

 ft rays and the line-spectrum of 7 rays are to be ascribed to 

 certain definite regions of vibration within the atom. It is 

 * J. Chadwick, Ber. d. D. Phys. Ges. xvi. p. 383 Q9U). 



