290 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



is independent of the applied voltage within the region A (see Ioniza- 

 tion Chambers, 5-21). 



region b: proportionality region. As the applied voltage is 

 raised the kinetic energy of the electrons and ions increases, so 

 that between collisions they will accumulate enough energy to create 

 new electrons and ions by inelastic impact with neutral molecules. 

 This process arises first in the close vicinity of the wire (anode) where 

 the field strength is highest. Each electron and ion formed by 

 collision may produce new ions; the current increases in the form 

 of an avalanche (Townsend avalanche). If n electron-ion pairs are 

 formed by the incident particle and if (A — 1) ion pairs are formed 

 by each initial electron on its way toward the anode, then A X n 

 ion pairs will migrate toward and be collected by the electrodes, and 

 the output signal will be 



AE = IcAn 



The size of the output pulse is, therefore, proportional to n, the 

 number of ions initially formed by the incident radiation. A is the 

 amplification factor; its value depends upon the applied voltage 

 as well as upon the wire radius, the capacitance of the counter 

 system, the gas pressure, and the kind of gas. The value of A is 

 between about 1 and 100 for proportional counters filled with raon- 

 atomic and diatomic gases and can rise to 10 4 for counters filled 

 with polyatomic gases. The discharge in the counter ceases when 

 all electrons and ions are collected at the electrodes. 



The magnitude of the proportional pulse can be as high as 10 to 

 100 mV. The pulse rises to a maximum value in a fraction of a 

 microsecond; the decay time depends upon the time constant of 

 the network following the counter. In general, the entire counting 

 process has a duration of several microseconds. After that time the 

 counter is ready for the recording of another pulse. 



The proportional counter permits the measurement of the number 

 of particles caused by primary ionization and, therefore, of the 

 energy of the ionizing particles. It is, furthermore, useful for the 

 observation of highly ionizing particles, such as a particles, in the 

 presence of a low ionizing radiation (e.g., y-ray background). 



REGION C: REGION OF LIMITED PROPORTIONALITY. As the applied 



voltage is further raised, the behavior of the counter changes 

 primarily in two respects : ( 1 ) For low-energy ionizing particles, the 

 size of the output pulse increases with increasing voltage faster than 

 in the proportional region, and (2) for high-energy ionizing particles, 

 the pulse size rises with increased applied voltage, but to a lesser 



