298 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



Point counters filled with argon or nitrogen and a quenching agent 

 have been investigated by Lion and Vanderschmidt. 1 The counting 

 rate increases first sharply with the applied voltage, as shown in 

 Fig. (5-2)17, region A . With further increasing voltage there follows 

 a region (B) of stable counting in which the counting rate increases 

 linearly with the voltage. At still higher voltages the counting rate 

 increases again rapidly (C), and the operation becomes unstable. 

 The quenching agent has the effect of broadening the region B as 



o 



1,400 1,600 1,800 2,000 2,200 

 Applied voltage 



Fig. (5-2)17. Characteristic of a point 

 counter. Counting rate as function of 

 the applied voltage (atmospheric pres- 

 sure, argon + 12 percent alcohol, point- 

 to-plane distance 0.1 in. [from K. S. 

 Lion and G. F. Vanderschmidt, J . Opt. 

 Soc. Am., 45, 1024 (1955); by per- 

 mission]. 



\ Radiation 



3^ 



\R 



-±E 



Fig. (5-2)18. Parallel -plate 

 counter. 



well as of shifting it to higher voltages. A plateau like that in cylin- 

 drical counters, Fig. (5-2)11, does not exist. The critical volume 

 (volume in which an electron must enter to trigger the discharge) 

 is considerably smaller than that of the cylindrical counter and is 

 commonly assumed to be located in a cone in front of the needle. 

 c. Parallel-plate Counters. The counter is shown schematically in 

 Fig. (5-2)18. The distance between the electrodes is usually a frac- 

 tion of a millimeter. One of the electrodes must be thin enough to 

 permit ionizing particles to enter the counter. The discharge mecha- 

 nism is initiated by the liberation of an electron from the counter 

 wall (only rarely by gas ionization). The electron forms an avalanche 

 and, at low voltages, an output pulse of the order of millivolts. As 

 the voltage is raised, spark discharges occur which can be observed 

 visually through a transparent electrode. The output pulses are of 



1 K. S. Lion and G. F. Vanderschmidt, J. Opt. Soc. Am., 45, 1024 (1955). 



