310 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



Chap. 10 



portional pulses are observed. It is often identical with the starting voltage, 

 depending upon the sensitivity of the detecting circuits. 



3. Geiger-Miiller threshold voltage. This is the lowest voltage at which 

 all pulses have the same height. 



4. Operating voltage. This is the normal voltage at which a counter is 

 operated. Usually it is in the lower half of the plateau region. 



5. Overvoltage. This is the difference between the operating voltage and 

 Geiger-Miiller threshold voltage. 



In a similar way, certain voltage ranges are distinguished by characteristic 

 behavior of the counter tube. The voltage ranges usually referred to are 

 given below (see Fig. 78). 



VOLTAGE 



Fig. 78. Ionization produced in counters as a function of anode voltage. 

 Montgomery and D. D. Montgomery, J . Franklin Inst., 231, 447 (1941).] 



[From C. G. 



1. Region of a few volts above ground potential within which some of the 

 positive and negative ions formed by an ionizing particle recombine before 

 reaching the anode or cathode. 



2. Ionization-chamber region, usually in the order of tens of volts. In 

 this region the initial ions formed by the primary particle are collected but 

 do not multiply appreciably. Recombination is negligible. 



3. Proportional region. The pulse produced by an ionizing event is 

 proportional to the initial number of ions formed for any fixed operating 

 voltage. 



4. Geiger-Miiller transition or limited proportionality region. Pulses 

 are no longer strictly proportional, and large pulses may exhibit some of the 

 characteristics of the Geiger-Miiller region. 



5. Geiger-Miiller region. Each event produces a discharge in which pulse 

 size is independent of initial intensity of ionization and the counting rate 

 remains essentially independent of voltage. 



