294 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



0.4 





 



incident radiation for which the counter is suitable can be extended 

 by operating the counter intermittently, i.e., with square pulses. 

 A discharge will then occur only if an ionizing particle traverses the 

 counter during the time when the square pulse is applied to the 

 counter. 1 The scale factor, i.e., the ratio of counting rate for con- 

 tinuous (d-c) operation to that of intermittent (pulse) operation, is 

 F s = llwf, where w is the pulse width and/ the pulse repetition rate. 

 Besides an extension of the useful range of the counter toward 

 higher levels of radiation, the pulsed operation offers two further 

 advantages: (1) The rate of spontaneous counts is reduced (Rossi 



and Staub describe a counter of 



p (~) 



0.125 in. diameter which is pulsed 

 six times per second and furnishes 

 not more than six spontaneous 

 counts per hour), and (2) the size 

 of the pulse is greatly increased 

 (on the order of 1,000 volts) be- 

 cause, for the duration of the 

 pulse, the voltage can be raised 

 above the sparking potential. 



Two curves representing the 

 average counter current versus 

 radiation intensity for d-c and 

 pulsed operation are shown in Fig. 

 (5-2)13. The linear range of the 

 average tube current versus incident radiation intensity is increased 

 without altering the performance of the tube at lower radiation 

 levels. 



The lowest level of radiation that can be measured with a Geiger 

 counter depends upon the background counting rate and upon the 

 error that can be tolerated. If the number of background counts in 

 any time interval t is C B , the number of counts resulting from 

 incident radiation (the radiation to be measured) is C s , and if C T 

 (= C s + C B ) is the number of total counts measured in the time 

 interval t, the statistical probable error of C T is 



10 10 2 10 3 

 1 1 1 liroentgen /hr 



10' 



Fig. (5-2)13. Characteristics (average 

 current versus incident radiation) of a 

 Geiger counter: (o) for d-c operation; 

 (b) for pulse operation [from S. W. 

 Lichtman, Nucleonics, 11, 22 (1953); 

 by permission}. 



e = 



VC T + C B VC S + 2C d 



c s c s 



If S is the rate at which ionizing particles of the radiation to be 

 measured enter the counter, r\ the counter efficiency, and B the 



1 B. B. Rossi and H. H. Staub, op. cit., p. 117; S. W. Lichtman, Nucleonics, 

 11, 22 (January, 1953). 



