﻿Straggling of a Particles by Matter. 415 



lull curve o£ fig. 1, where ionization is plotted as ordinate 

 and distance from the radioactive source as abscissa. Now 

 it has been shown experimentally that the (average) velocity 

 of the a particles at any point of their path is proportional 

 to the cube root of their remaining range. Assuming that 

 the ionization produced is proportional to the energy lost by 

 the a particle at any point of its path, it can at once be 

 shown that the ionization should be inversely proportional 

 to the cube root of the remaining range. Such a theoretical 

 ionization curve is shown as the dotted curve of fig. 1. It 



Fis-. 1. 



Rcinge 



will be seen to be in approximate agreement with experiment 

 over the first portion of the path of the a particle, but as the 

 maximum is approached this agreement fails. 



G-eiger * has suggested that the ionization curve observed 

 for a beam of a rays should be different from that of a single 

 a particle, owing to slight variations in the ranges of the 

 latter, i. e. to straggling. The ionization curve, built up of 

 a large number of theoretical curves grouped around one ot 

 average range, will thus be modified considerably near the 

 maximum where the ionization is changing rapidly Hence 

 the shape of the ionization curve near the end of the range 

 should give an indication of the amount of straggling. 

 * Geiger, Proc. Roy. Soc. A. lxxxiii. p. 505 (1910). 



