ASSAY OF RADIOACTIVITY 



electrodes, (2) for an initial ionization involving a much larger number of 

 ion pairs, as might be formed by an a-particle. Starting from zero potential, 

 there is first a region (A) in which the collected charge increases rapidly with 

 voltage. At these relatively low potentials (up to about 50 V) only some of 

 the electrons are collected, the remainder being lost by re-combination with 

 positive ions before they have time to reach the anode. The fraction collected 

 rises until a point is reached where the members of every ion pair are separated 

 fast enough to prevent their re-combination. Above this potential there is 



0) 0) 



!9 



c 



dJ 



£ -Si 

 ?8 



200 AOO 600 



Anode voltage 



800 



1000 



1200 



Figure 31.1 The variation with applied voltage of the charge collected at the 

 anode of a gas chamber — -for explanation of symbols see text 



a plateau region {B) where collection is complete, and where the current is 

 directly proportional to the number of ion pairs liberated within the sensitive 

 volume in unit time. In this 'saturation' region the charge is nearly indepen- 

 dent of the applied voltage, but varies considerably with the nature of the 

 incident radiation, as indicated by the difference between curves 1 and 2. 

 If the electrode potential is further increased some of the electrons acquire 

 sufficient kinetic energy while being accelerated towards the anode to be 

 able to produce additional ion pairs in their turn, this process being known as 

 gas multiplication. In region (C), the zone in which proportional counters 

 are operated, the multiplication factor increases somewhat with voltage, 

 but at a given voltage each primary ion pair is multiplied to the same extent, 

 so that curves 1 and 2 rise in parallel, and the amount of charge collected 

 is still dependent on the nature of the incident radiation. There foffows a 

 transition region (Z)) in which the multiplication factor rises still further, 

 and in which the number of electrons reaching the anode ceases to be pro- 

 portional to the number liberated in the initial ionization. At a sufficiently 

 high voltage the Geiger-Muller region {E) is reached. Here curves 1 and 2 

 coincide, a single ion pair causing an avalanche of electrons to reach the 

 anode whose magnitude depends only on the characteristics of the chamber. 

 Still higher voltages result in a continuous discharge (F). 



Figure 31.1 is purely diagrammatic, and the exact lengths of the various 

 regions and the slopes of the curves depend in a complicated way on the 



424 



