Sec. 5-2] 



RA DIA TIOX TIL 1 NS DUC E RS 



289 



^ ^Radiation 



n 



r 1 



Anode 



PI 



^1 



Cathode 



LJ 



.if 



6> ,A^ 



+ 



Fig. (5-2)9. Geiger-Miiller counter and 

 basic circuit. 



discharge will take place in the counter. As the result of this dis- 

 charge, an intermittent current will How and a voltage pulse 

 will appear across the resistance R. The current can be many 

 orders of magnitude larger (up to 10 9 ) than the current caused by 

 the movement of the electrons and 

 ions formed by the ionizing par- 

 ticle. 



Four different modes of opera- 

 tion of the counter can be distin- 

 guished, depending primarily upon 

 the magnitude of the voltage ap- 

 plied to the counter: 



region a: ionization chamber 

 region, fig. (5-2)10. An ionizing 

 particle, such as an a or j3 particle, 



entering the tube will produce electrons and ions. If a voltage is 

 applied, the charges will be separated; the electrons will drift to- 

 ward the anode, and the positive 

 ions toward the cathode. If the 

 voltage applied is high enough to 

 move the electron sand ions toward 

 the electrodes before an appreci- 

 able fraction of them recombine, 

 yet not so high that ionization by 

 collision occurs, the number of 

 charges moving toward and arriv- 

 ing at the electrodes is equal to the 

 number produced by the initial 

 ionizing event. The migration of 

 the electrons and ions toward the 

 electrodes causes a current and the 

 appearance of an output signal 

 across the resistor R, which is pro- 

 portional to the number of ions n 

 formed by the incident ionizing 

 radiation : 



AE = kn 



The counter acts as an ioniza- 

 tion chamber. The size of the pulse 



500 1,000 



Applied voltage, volts 



,500 



^Counting rate; 

 number of large 

 (Geiger) pulses 

 per second 



500 1,000 1,500 



Applied voltage, volts 



Fig. (5-2)10 (top). Operating character- 

 istics of counters: .4, region of ioniza- 

 tion chamber; B, region of proportion- 

 ality; C, region of limited proportion- 

 ality; 1), Geiger region; E, region of 

 unstable operation. 



Fig. (5-2)11 (bottom). Counting rate 

 versus applied voltage of a Geiger 

 counter. 



