XVII. 



RADIOACTIVE TRACERS 



017 



process inside the counter and is independent of the number of ions 

 produced by the original particle. Thus no discrimination can be 

 made among the various types of radiation by the size of the pulse 

 (38). If one plots a curve of the observed counting rate foi- a given 

 source vs. the voltage, the curve has the characteristic shape shown in 

 Figure 6. The level portion, known as a plateau, is the operating 

 region for the Geiger counters. The length and slope of the plateau 

 are important characteristics of the tube. 



(leiger counters are usualty divided into two types, depending on 

 the nature of the process by which the discharge is stopped. If the 



800 900 1000 1100 1200 



POSITIVE HIGH VOLTAGE APPLIED TO TUBE 



Fig. G. Counting rate r.s. voltage — "plateau" curve for Geiger counter. 



gas contains about 10% of an organic vapor such as ethyl alcohol, di- 

 ethyl ether, an ester, or a hydrocarbon along with 90% of a rare gas 

 (argon or neon), the stopping of the discharge is aided by intermediate 

 steps that lead to disruption of the organic molecules. Such a counter 

 is said to be self-quenching. If the gas does not contain an organic 

 vapor no such mechanism will help and an external electronic circuit is 

 needed to lower- the potential across the counter rapidly after the dis- 

 charge starts. Such a circuit is the typical Neher-Harper circuit 

 shown in Figure 7b {29). 



In the firing of a Geiger counter the potential drops as a result 

 of the current carried by the original and secondary ions across the 

 liigh voltage. This current causes the potential to drop, either 

 unassisted in llie conventional resistance-capacitance coupling of a 

 self-quencliing counter (Fig. 7a) or with the help of a Xeher-Harper 



