Sec. 5-2] 



RADIA TION TRANSDUCERS 



293 



the plateau rises with <1 per cent/100 volts. At the end of the 

 plateau the counting rate increases again, and the counter may pro- 

 duce self-sustained, continuously repeating pulse discharges even in 

 the absence of external ionizing radiation. Operation of the counter 

 in this region, Fig. (5-2)102?, is likely to damage the counter. 



Figure (5-2)1 1 is not to be interpreted in the sense that the number 

 of counts below the starting potential E s is zero. Rather, a large 

 number of small pulses can be observed at voltages below E g , pro- 

 portional pulses of different sizes, but smaller than (Jeiger pulses. 

 The number of proportional pulses counted depends upon the sensi- 

 tivity (discriminator setting) of the subsequent amplifier stage. 



If an electron or ion pair is produced by incident ionizing radia- 

 tion, the counter will start to respond after a very short time delay 

 ^ , as shown in Fig. (5-2)12. This 

 time delay is due to the electron 

 transit time; it is of the order of 

 a fraction of a microsecond and 

 may exceed 1 ^sec only in large 

 counters. The output pulse rises 

 to its maximum value in a few 

 microseconds and decays within 

 several hundred microseconds. 

 During this time the counter does 

 not register any ionization caused 



by a new incident particle or radiation (dead time). After that 

 time there follows a period (recovery time) where the counter will 

 respond with small pulses; toward the end of the recovery time 

 the pulse increases to normal size. 1 Also, the recovery time is of 

 the order of several hundred microseconds. It is shorter in non- 

 self-quenching counters than in those filled with a quenching agent. 



The highest level of radiation for which a Geiger counter is suit- 

 able is determined by the recovery time of the counter. In com- 

 mercial counters, this time is of the order of 30 to 300 /isec; the 

 highest counting rate is, therefore, 3 X 10 3 to 3 x 10 4 cps. 



In the plateau region, the number of counts is proportional to the 

 number of primary ionizations provided the ionizing particles arrive 

 at a sufficiently low rate. As the number of incident ionizing par- 

 ticles or the incident radiation rises, the counter cannot follow any 

 longer; the number of counts, as well as the average output current, 

 reaches a maximum and then declines with further increase of 

 incident radiation. The situation can be remedied, and the region of 



1 H. G. Stever, Phys. Rev., 61, 38 (1942). 



Fig. (5-2)12. Pulse height-time dia- 

 gram of a Geiger counter. 



