PHOTOELECTRIC DETECTORS 



liberation of secondary electrons. Thus an amplification of the primary 

 photocurrent occurs; the anode potential must exceed a critical value but 

 not be so high as to produce irreversible total ionization of the gas. By gas 

 filling it is possible to achieve an ampHfication of 10 or more over a reasonable 

 range of light intensity. In this way sufficient current may be available to 

 work relays, etc., with little additional thermionic valve amplification. 



The sensitivity of a vacuum cell is independent of the anode potential, 

 providing it is higher than the saturation potential of the cathode, so that 

 close control of anode voltage is usually unnecessary ; but the amplification 

 factor of a gas filled cell does depend on the anode voltage and tight control 

 is required if a constant sensitivity is desired. 



Gas filled cells have a poor frequency response. Sensitivity falls off 

 appreciably above 1,000 c/s. The frequency response of a vacuum cell can 

 extend to many megacycles per sec if care is taken in the design to keep 

 inter-electrode capacity low. 



1^^ 28 mnn, 

 max 



^ 32mm 

 max 



Figure 28.26 Dimensions of Mullard type 20CG and 20CV photoemissive cell 



The number of different size photoemissive cells commercially available is 

 very large indeed and reference to manufacturers' data sheets is required in 

 order to select a suitable cell for a particular need. Two examples of a 

 popular size photoemissive cell will be given as they illustrate the performance 

 of this group of detectors. 



The Mullard type 20CV is a vacuum photoemissive cell with the dimen- 

 sions shown in Figure 28.26 and type 20CG is a gas filled cell of equivalent 

 dimensions. Both these cells have a type SI photocathode so that they are 

 particularly suitable for use with an incandescent light source or for the 

 detection of radiation out to 1-1 ^. No satisfactory photomultiplier cell has 

 been marketed with this type of spectral response. 



Figure 28.27 shows the performance of an average 20CV cell under a 

 variety of illumination levels. It can be seen that, for a given light flux, 

 saturation occurs with an anode voltage of the order of 25 V and that an 

 increase in anode voltage above this level has little effect on the photocurrent. 



359 



