﻿44 Mr. Gr. EL. Henderson on the 



Secondly, a more direct measure of the amount o£ 

 straggling can be determined by counting the number of 

 u particles at different points along the patli of a parallel beam. 



It is proposed in this paper to discuss the theoretical and 

 experimental data on straggling, and it will be shown that 

 the observed amount of straggling is much in excess of that 

 allowed by theory. Further experimental evidence bearing 

 on straggling will also be brought forward. 



§ 2. The Straggling in Air. 



It might be thought that the individual a particles are 

 emitted with slightly different velocities, thus giving rise to 

 straggling. It has been shown by Geiger (loc. cit.), however, 

 that the a particles emitted from a thin layer of radioactive 

 material do not differ by as much as -J per cent, in initial 

 velocity. Thus the cause of the straggling must be looked 

 for in the air itself. 



As the a particle passes through the air it gives up its 

 energy to the electrons and nuclei of the air atoms, and it is 

 occasionally deflected through a considerable angle by close 

 encounters with the nuclei. Different a. particles will 

 encounter different numbers and distributions of electrons 

 and nuclei and accordingly are straggled out. The calcu- 

 lation of the consequent probability variations in the ranges 

 of the individual a. particles has been carried out by both 

 Bohr * and Flamm |- They agree in showing that the 

 nuclei produce practically no straggling. They also agree 

 closely in the amount of straggling produced by the electrons. 

 The straggling of various types of a rays in air, calculated 

 by Flamm's method, is shown in the second column of 

 Table I. The value tabulated is the distance, measured 

 along the range, over which the number of particles in a 

 parallel beam falls off from '92 to 'OS of the original number. 

 This corresponds approximately to the method of measuring 

 the straggling from the experimental curves. 



The ionization curves for three types of a. rays have 

 recently been determined with some accuracy by the writer %. 

 The full curve given in fig. 1 is a reproduction of the 

 ionization curve found for RaC. It was shown that the 

 ionization curve from C to B (fig. 1) could be represented 

 very approximately by a straight line. The slope of this 

 straight line furnishes information as to the magnitude of 



* Bohr, Phil. Mag. xxx. p. 581 (1915). 



+ Flarnm, Wien. Ber. II a. cxxii. p. 1393 (1913). 



X Henderson, Phil. Mag. xlii. p. 538 (1921). 



