Sec. 5-2] RADIATION TRANSDUCERS 291 



degree. Therefore, in the region C, the output pulse is not propor- 

 tional any more to the initial ionization (number of ion pairs formed 

 by the ionizing particle), until at the upper limit of the region of 

 limited proportionality all pulses have the same size regardless of 

 whether they are initiated by one or many ion pairs. The amplifi- 

 cation factor can rise as high as 10 7 . Counters are rarely used in the 

 region of limited proportionality; the description of the physical 

 mechanism is omitted, therefore. 



region d. geiger-counter region. The characteristic property 

 of a counter operated in the Geiger region is the appearance of out- 

 put pulses that are all of the same size, independent of the number 

 of ions initially formed by an incident ionizing particle or radiation. 



The physical mechanism which leads to the formation of a Geiger 

 pulse is initially the same as that giving rise to a proportional pulse: 

 An incident ionizing particle produces an electron-ion pair. The 

 electron drifts toward the positive central wire and, within a narrow 

 region around it, causes ionization by collision and the formation 

 of an avalanche. 



Before the electrons and ions formed in the avalanche are col- 

 lected at the electrodes, a second phenomenon takes place. Light 

 emitted from excited atoms in the first avalanche causes photo- 

 ionization of the gas molecules and the liberation of new electrons 

 which cause further avalanches. The light emission proceeds in 

 every direction, but only in the vicinity of the central wire is the 

 field strength high enough to facilitate the formation of new ava- 

 lanches. The discharge spreads along the wire, therefore, until the 

 entire anode is surrounded by a narrow cylinder of ions, and the 

 discharge becomes self-sustaining. 



The termination of the discharge comes about in the following 

 manner. The positive ions surrounding the central anode move 

 toward the cathode with a velocity which is considerably smaller 

 (because of their mass) than that of the electrons. These ions cause 

 a positive space charge surrounding the positive wire; as they move 

 toward the cathode, the positive space-charge cylinder ("the virtual 

 anode") increases in diameter and, therefore, the field strength in 

 the counter decreases. [The field strength at a radius r between the 

 cylinder radius r c and the wire radius r a is F = Ejr log [rjr a ).] The 

 discharge ceases when the field strength within the counter has 

 fallen below the level where a discharge can be sustained. 



The pulse size of Geiger counters is, in general, between 1 and 10 

 volts and sometimes higher; the amplification factor is between 10 7 

 and 10 9 . 



