IONIZATION AND DETECTION 



109 



KZ3> 



©-v 



© 



t 



(a) 



0> 



' phot 



00 



photons 



to 

 • voltmeter 



Geiger 



threshold 



saturation (s) 

 region 



Figure 5-3. Ionization Chamber: (a) sche- 

 matic design — wire anode, A, and cylindrical 

 cathode, K, filled with gas (e.g., Argon); (b) 

 charge collected at A per pulse at different 

 voltages. (See text.) 



"avalanche" process. The multiplication factor may be as high as 10 3 or 10 4 , 

 so that the current pulse which is produced may be 10 3 or 10 4 times larger 

 than the "saturation" pulse (Figure 5-3 (b)). Since the pulse size is propor- 

 tional to the energy lost by the original photon or particle, a chamber oper- 

 ated in this fashion is known as a "proportional" counter. 



At higher voltages, the multiplication factor for large pulses tends to be 

 smaller than that for small pulses, and all pulses are multiplied to a constant 

 size regardless of initial strength. The voltage at which this gaseous dis- 

 charge starts to occur is known as the "Geiger threshold." 



Figure 5-3 shows an ion-chamber design from which the proportional 

 counter and the Geiger counter may be developed. Figure 5-4 is a photo- 

 graph of a typical unit. 



(3) Fluorescence Induced in Solids and Liquids: The light emitted after the ab- 

 sorption of ionizing radiation by a fluorescent solid is reflected on to the 



