﻿Straggling of a Particles by Matter. 



45 



the straggling. The. easiest way of considering the matter 

 is to imagine the straight line produced in both directions 

 till it meets the axes or! zero and maximum ionization at D 

 and E. Then the projection of the line DE on the axis of 

 zero ionization (or the reciprocal of its slope) is a direct 

 measure of the straggling. The greater the straoro-lino- the 

 greater will be the projection referred to, and as a first 

 approximation the projection may be taken as proportional 

 to the straggling. The values of the projections taken from 

 the writer's curves are given in the third column of Table I. 

 That for polonium has been determined from the curves 

 given by Lawson *. 



The curves obtained by counting the number of a particles 

 in a parallel beam at various points along the path show 

 that this number remains constant till near the end of the 

 range and then falls off rapidly to zero. Most of this falling 

 off also approximately follows a straight line. The reciprocal 

 slope or projection of this line is a more direct measure of 

 the straggling than the corresponding projection of the 

 ionization curve. Measurement of the straggling by means 

 of counting experiments is, however, very slow, as large 

 numbers of a particles must be counted. The values 

 obtained from the most recent and reliable counting experi- 

 ments are given in the fourth column of Table I. The 

 result for polonium is taken from a scintillation curve given 

 by Rothensteiner f ; that for RaC is from a curve obtained 

 by Makower { by photographic counting of the a particles. 

 All the results in the Table are in millimetres and refer to 

 air at 0° C. and 760 mm. 



Table I. 



Type of Kays. 



Theoretical 

 Straggling. 



Straggling from 

 Ionization Curves. 



Straggling from 

 Counting Expts. 





•88 

 102 

 144 

 1-74 



3T 40 



; Thorium G x 



2-88 

 2-83 

 2-92 



4T 



: Thorium C 2 



This Table shows clearly that the observed values of the 



* Lawson, Wien. Her. II a. cxxiv. p. 637 (1,915). 



t Rothensteiner, Wien. Ber. II a. cxxv. p. ]237 (1915). 



% Makower, Phil. Mag. xxxii. p. 222 (1916). 



