GENERATION, CONTROL, AND MEASUREMENT 233 



the number of photoelectrons emitted by the photocathode is propor- 

 tional to the intensity over many orders of magnitude. At constant flux 

 intensity the photocurrent at first rises rapidly with applied voltage and 

 then saturates in the region of 20-30 v (Fig. 3-30). In the region of 

 saturation the current does not appreciably increase with voltage and is 

 therefore relatively insensitive to small changes in applied voltage. The 

 combination of linearity and insensitivity to small voltage changes makes 



40 



160 



200 



80 120 



ANODE POTENTIAL, V 



Fig. 3-30. Current-voltage characteristics of vacuum (solid curves) and gas (dashed 

 curves) photoemission cells at two values of incident flux. 



the high-vacuum photocell especially useful for precise quantitative 

 measurement. 



The gas photoemission cell contains a readily ionizable gas, such as 

 argon, at low pressure. Inelastic collisions between the electrons and 

 gas molecules result in the formation of ions that cause a large increase 

 in the current. The use of an ionizable gas at low pressure was one of 

 the first methods of internally amplifying the photoelectric current. Gas 

 amplification factors of 5 to 15 can be obtained with commercial gas cells, 

 and stable operation still be maintained. The principal disadvantages 

 of the gas photocell are that (1) the response is no longer a Hnear func- 

 tion of intensity if the voltage is high enough to produce appreciable 

 amplification ; (2) the sensitivity is markedly influenced by applied volt- 

 age, as shown by the curves of Fig. 3-30; and (3) the gas cell has a rela- 

 tively high noise level under most conditions of operation. The gas 

 photocell is used principally for nonquantitative applications, such as in 

 the operation of relays where linearity and constancy of behavior are not 



important. 



The photomultiplier tube is a high-vacuum photocell containing an 

 internal current-amplifying system (Engstrom, 1947; Marshall e\ al. 



