292 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



The discharge will be reignited if the positive ions arrive at the 

 cathode and liberate new electrons. To avoid this possibility, three 

 different methods of "quenching" the counter are available. 



1. Resistance quenching. The capacitance of the counter is 

 expressed by the equivalent capacitor C in Fig. (5-2)9. The gas-dis- 

 charge process reduces the voltage on this capacitor; the capacitor 

 is recharged through the resistor R. If the time constant CR is 

 made longer than the time required to sweep the ions out of the 

 counter, no reignition will take place. The method requires time 

 constants of the order of 10~ 2 sec; during this time the counter is 

 inoperative, and the method cannot be used for fast counting. 



2. Electronic-quenching methods. A number of systems have 

 been devised in which the output pulses from the counter cause an 

 electronic circuit to reduce or remove the voltage on the counter 

 temporarily and then to recharge the counter. The systems can be 

 made to operate in about 10~ 4 sec. 1 



3. Self- quenching counters. The admixture to the inert gas in 

 the counter of a small amount (about 10 per cent) of a polyatomic 

 gas (i.e., generally a gas with four and more atoms, frequently a 

 hydrocarbon such as methane or alcohol vapor) changes the quench- 

 ing characteristic of the counter profoundly and prevents a reigni- 

 tion of the discharge. The photons emitted from the monatomic or 

 diatomic gas interact with the polyatomic gas. Thus, these photons 

 are prevented from reaching the cathode where they may liberate 

 new electrons. Instead, they are absorbed by the heavy organic 

 molecule, which is either predissociated or decomposed. If the 

 energy is finally carried to the cathode surface, it is too small to 

 cause a photoelectric emission. Counters filled with such gases 

 usually have a higher starting potential than non-self-quenching 

 counters, their lifetime is shorter because of the decomposition of 

 the quenching agent in each discharge, and they show variation 

 of their behavior at low temperatures, where condensation of the 

 quenching agent may occur. 



If the counter is exposed to ionizing radiation and the voltage 

 across the counter is raised, the counting rate will first increase, as 

 in Fig. (5-2) 1 1 . Then follows a region, the plateau, where the count- 

 ing rate will only slightly increase. This is the region in which the 

 counter should be operated. In the plateau region the number of 

 discharges is directly related to the number of ionizing particles 

 striking the tube and does not depend appreciably upon the applied 

 potential. No counter has an ideal plateau of a slope zero; at best 



1 See Korff, op. cit., chap. 7B. 



