Sec. 5-2] RA DIA TION TEA AN 1)1 CERS 2S.'{ 



For a steady flux of ionizing radiation, and if the losses of charges 

 within the chamber can be neglected, the saturation current is 



/ = N 8e 



where N Q is the number of particles arriving in the sensitive volume 

 of the chamber per second and causing ionization, S is the number of 

 ion pairs formed per ionizing particle, and e the electron charge. 

 The current can be measured with a galvanometer, but is in general 

 too small for this instrument. Measurement of the voltage drop 

 across a large resistance or electro metric methods are usually 

 employed. A convenient way to measure very small radiation inten- 

 sities consists in charging the ionization chamber to a voltage E v 

 Fig. (5-2)2, and exposing it to radiation for a time t. The chamber 

 will discharge to a voltage E 2 which should still be above E s , the 

 voltage at which saturation occurs. If E x and E 2 are measured 

 electrometrically, i.e., without drawing current from the charged 

 chamber, the average current during the time t can be found from 



r C(E X - E 2 ) 



where C is the capacitance of the ionization chamber (integrating- 

 chamber method). 



The sensitivity of the ionization chamber increases with the num- 

 ber of ion pairs formed per incident ionizing particle and decreases 

 with the number of ions lost during their travel through the chamber. 



The number of ion pairs formed per incident ionizing particle is, 

 approximately, 



n =Slp (1) 



where 8 is the specific ionization (number of ions per centimeter 

 of length and per atmosphere of pressure), I the average path length 

 of the ionizing particles through the chamber, and p the pressure in 

 atmospheres. 



The proportionality of the current with pressure as expressed by 

 Eq. (1) is correct only in the range below 10 atm. At higher pressure 

 the sensitivity increases at a reduced rate, and in the range above 

 20 to 200 atm (depending upon the field strength) the sensitivity 

 of the chamber decreases, because the mobilities of the ions are 

 reduced and recombination is increased. 1 



1 H. A. Erickson, Phijs. Rev. (1) 27, 473 (1908). 



