Sec. 5-2] 



RA DIA TIOX TRANSD UCERS 



285 



2. Electron pulse chamber. The time constant RC is larger than 

 the time required to collect the electrons but small compared to the 

 collection time for the ions (order of RC is 10 to 20 //sec). The 

 pulse height in this case is 



c 



E P - 



Since the total number of negative charges is close to that of 

 positive charges, the electron pulse chamber also furnishes informa- 

 tion as to the total number of ions produced in the chamber. This 



■±rE 



C± E-AE 



i 



1 



■ R 



AE,E 



T 



Fig. (5-2)3. Equivalent cir- 

 cuit of an ionization chamber. 



Fig. (5-2)4. Ionization chamber with 

 built-in grid. 



chamber is faster, but has the disadvantage that the pulse height 

 depends upon the location within the counter where the charges 

 are produced. By the insertion of a grid between the two electrodes, 

 as in the Frish chamber, Fig. (5-2)4, this difficulty can be overcome. 

 The grid is held at a potential between those of the electrodes. Elec- 

 trons produced between the negative electrode and the grid will 

 pass through the grid and will be recorded; the influence of the ion 

 movement upon the collector is shielded by the grid. 



The energy of an ionizing particle that spends its whole energy 

 within the chamber can be found from 



Q 



w 



where Q is the charge induced in the collector, and W is the energy 

 required for the formation of an ion pair (about 35 eV for alpha rays 

 in air). 



The mean square fluctuation of the output voltage is approxi- 

 mately (within a factor of 2) 



Atf 2 



E~ me 



RH 



me 



where E is the output voltage, / the current through the resistor R. 

 m the average number of ion pairs produced by each particle in the 



