308 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 10 



This process is possible only when the ionization potential of the quenching 

 gas is less than that of the nonquenching gas [4], The photons emitted in 

 the process are absorbed in turn by other polyatomic molecules. (2) The 

 quenching-gas ions, formed either by the initial ionizing event or by electron 

 exchange, are neutralized by electron ejection from the wall, but they exhibit 

 a greater probability for decomposition than for ejection of a free electron or 

 for radiative capture. Altogether, the probability of secondary electron 

 production appears to be 1 electron per 10 10 ions [6,7] and these can be 

 absorbed by the slow-moving residual quenching-gas ions. 



The presence in a counter tube of appreciable quantities of negative ion- 

 forming gases such as 2 , H 2 0, and the halogens considerably alters the 

 mechanism of discharge and with it the desirable operating characteristics. 



VOLTAGE 



Fig. 75. Effects of gases on G-M plateau. 



274 (1944).] 



A. Pure methane 



B. 1.5 cm argon added 



C. 1.5 cm water added 



VOLTAGE 

 [S. A. Korff and R. D. Present, Phys. Rev., 66, 



A. Pure methane 



B. 1.5 mm air added 



C. 6.0 mm air added 



The slow collection time of negative ions compared with electrons tends to 

 prolong the discharge time and frequently leads to the complete disappearance 

 of the Geiger-Miiller plateau [4], as seen in Fig. 75. 



10.4. Pulse and Voltage Characteristics. Voltage pulses as observed with 

 an oscilloscope increase rapidly to a maximum value in a time of the order of 

 1 microsecond, corresponding to the collection time of the electrons. Follow- 

 ing the pulse maximum or pulse rise time, the anode voltage, which has 

 actually been depressed, slowly recovers to normal potential as the residual 

 positive ions are swept out. During the first part of the pulse, referred to 

 as the dead time, the counter is wholly insensitive to a second ionizing event. 

 As the voltage recovers beyond a critical value, a second ionizing event may 

 produce at successively later intervals a larger pulse until, at the termination 

 of the recovery time, a normal pulse voltage is again produced, as shown in 

 Fig. 76. Both the dead time and recovery time appear to have durations 

 of about 10 -4 sec or less for self-quenching counters. 



Voltage characteristics of counters are determined by observing the 



